diff --git a/src/JSInterop/Microsoft.JSInterop/ref/Microsoft.JSInterop.csproj b/src/JSInterop/Microsoft.JSInterop/ref/Microsoft.JSInterop.csproj
index 52192f2100..dabf98ff45 100644
--- a/src/JSInterop/Microsoft.JSInterop/ref/Microsoft.JSInterop.csproj
+++ b/src/JSInterop/Microsoft.JSInterop/ref/Microsoft.JSInterop.csproj
@@ -3,6 +3,7 @@
   <PropertyGroup>
     <TargetFrameworks>netstandard2.0;$(DefaultNetCoreTargetFramework)</TargetFrameworks>
     <TargetFrameworks Condition="'$(DotNetBuildFromSource)' == 'true'">$(DefaultNetCoreTargetFramework)</TargetFrameworks>
+    <Nullable>annotations</Nullable>
   </PropertyGroup>
   <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0'">
     <Compile Include="Microsoft.JSInterop.netstandard2.0.cs" />
diff --git a/src/JSInterop/Microsoft.JSInterop/ref/Microsoft.JSInterop.netcoreapp.cs b/src/JSInterop/Microsoft.JSInterop/ref/Microsoft.JSInterop.netcoreapp.cs
index 3abe22eea0..23eee87818 100644
--- a/src/JSInterop/Microsoft.JSInterop/ref/Microsoft.JSInterop.netcoreapp.cs
+++ b/src/JSInterop/Microsoft.JSInterop/ref/Microsoft.JSInterop.netcoreapp.cs
@@ -30,7 +30,8 @@ namespace Microsoft.JSInterop
     public abstract partial class JSInProcessRuntime : Microsoft.JSInterop.JSRuntime, Microsoft.JSInterop.IJSInProcessRuntime, Microsoft.JSInterop.IJSRuntime
     {
         protected JSInProcessRuntime() { }
-        protected abstract string InvokeJS(string identifier, string argsJson);
+        protected abstract string? InvokeJS(string identifier, string? argsJson);
+        [return: System.Diagnostics.CodeAnalysis.MaybeNullAttribute]
         public TValue Invoke<TValue>(string identifier, params object[] args) { throw null; }
     }
     public static partial class JSInProcessRuntimeExtensions
@@ -42,15 +43,16 @@ namespace Microsoft.JSInterop
     {
         public JSInvokableAttribute() { }
         public JSInvokableAttribute(string identifier) { }
-        public string Identifier { [System.Runtime.CompilerServices.CompilerGeneratedAttribute] get { throw null; } }
+        public string? Identifier { [System.Runtime.CompilerServices.CompilerGeneratedAttribute] get { throw null; } }
     }
     public abstract partial class JSRuntime : Microsoft.JSInterop.IJSRuntime
     {
         protected JSRuntime() { }
         protected System.TimeSpan? DefaultAsyncTimeout { [System.Runtime.CompilerServices.CompilerGeneratedAttribute] get { throw null; } [System.Runtime.CompilerServices.CompilerGeneratedAttribute] set { } }
         protected internal System.Text.Json.JsonSerializerOptions JsonSerializerOptions { [System.Runtime.CompilerServices.CompilerGeneratedAttribute] get { throw null; } }
-        protected abstract void BeginInvokeJS(long taskId, string identifier, string argsJson);
+        protected abstract void BeginInvokeJS(long taskId, string identifier, string? argsJson);
         protected internal abstract void EndInvokeDotNet(Microsoft.JSInterop.Infrastructure.DotNetInvocationInfo invocationInfo, in Microsoft.JSInterop.Infrastructure.DotNetInvocationResult invocationResult);
+        [System.Diagnostics.DebuggerStepThroughAttribute]
         public System.Threading.Tasks.ValueTask<TValue> InvokeAsync<TValue>(string identifier, object[] args) { throw null; }
         public System.Threading.Tasks.ValueTask<TValue> InvokeAsync<TValue>(string identifier, System.Threading.CancellationToken cancellationToken, object[] args) { throw null; }
     }
@@ -74,7 +76,7 @@ namespace Microsoft.JSInterop.Infrastructure
     {
         public static void BeginInvokeDotNet(Microsoft.JSInterop.JSRuntime jsRuntime, Microsoft.JSInterop.Infrastructure.DotNetInvocationInfo invocationInfo, string argsJson) { }
         public static void EndInvokeJS(Microsoft.JSInterop.JSRuntime jsRuntime, string arguments) { }
-        public static string Invoke(Microsoft.JSInterop.JSRuntime jsRuntime, in Microsoft.JSInterop.Infrastructure.DotNetInvocationInfo invocationInfo, string argsJson) { throw null; }
+        public static string? Invoke(Microsoft.JSInterop.JSRuntime jsRuntime, in Microsoft.JSInterop.Infrastructure.DotNetInvocationInfo invocationInfo, string argsJson) { throw null; }
     }
     [System.Runtime.InteropServices.StructLayoutAttribute(System.Runtime.InteropServices.LayoutKind.Sequential)]
     public readonly partial struct DotNetInvocationInfo
@@ -92,11 +94,11 @@ namespace Microsoft.JSInterop.Infrastructure
     {
         private readonly object _dummy;
         private readonly int _dummyPrimitive;
-        public DotNetInvocationResult(System.Exception exception, string errorKind) { throw null; }
-        public DotNetInvocationResult(object result) { throw null; }
-        public string ErrorKind { [System.Runtime.CompilerServices.CompilerGeneratedAttribute] get { throw null; } }
-        public System.Exception Exception { [System.Runtime.CompilerServices.CompilerGeneratedAttribute] get { throw null; } }
-        public object Result { [System.Runtime.CompilerServices.CompilerGeneratedAttribute] get { throw null; } }
+        public DotNetInvocationResult(System.Exception exception, string? errorKind) { throw null; }
+        public DotNetInvocationResult(object? result) { throw null; }
+        public string? ErrorKind { [System.Runtime.CompilerServices.CompilerGeneratedAttribute] get { throw null; } }
+        public System.Exception? Exception { [System.Runtime.CompilerServices.CompilerGeneratedAttribute] get { throw null; } }
+        public object? Result { [System.Runtime.CompilerServices.CompilerGeneratedAttribute] get { throw null; } }
         public bool Success { [System.Runtime.CompilerServices.CompilerGeneratedAttribute] get { throw null; } }
     }
 }
diff --git a/src/JSInterop/Microsoft.JSInterop/ref/Microsoft.JSInterop.netstandard2.0.cs b/src/JSInterop/Microsoft.JSInterop/ref/Microsoft.JSInterop.netstandard2.0.cs
index 3abe22eea0..b62ccbb677 100644
--- a/src/JSInterop/Microsoft.JSInterop/ref/Microsoft.JSInterop.netstandard2.0.cs
+++ b/src/JSInterop/Microsoft.JSInterop/ref/Microsoft.JSInterop.netstandard2.0.cs
@@ -30,7 +30,7 @@ namespace Microsoft.JSInterop
     public abstract partial class JSInProcessRuntime : Microsoft.JSInterop.JSRuntime, Microsoft.JSInterop.IJSInProcessRuntime, Microsoft.JSInterop.IJSRuntime
     {
         protected JSInProcessRuntime() { }
-        protected abstract string InvokeJS(string identifier, string argsJson);
+        protected abstract string? InvokeJS(string identifier, string? argsJson);
         public TValue Invoke<TValue>(string identifier, params object[] args) { throw null; }
     }
     public static partial class JSInProcessRuntimeExtensions
@@ -42,15 +42,16 @@ namespace Microsoft.JSInterop
     {
         public JSInvokableAttribute() { }
         public JSInvokableAttribute(string identifier) { }
-        public string Identifier { [System.Runtime.CompilerServices.CompilerGeneratedAttribute] get { throw null; } }
+        public string? Identifier { [System.Runtime.CompilerServices.CompilerGeneratedAttribute] get { throw null; } }
     }
     public abstract partial class JSRuntime : Microsoft.JSInterop.IJSRuntime
     {
         protected JSRuntime() { }
         protected System.TimeSpan? DefaultAsyncTimeout { [System.Runtime.CompilerServices.CompilerGeneratedAttribute] get { throw null; } [System.Runtime.CompilerServices.CompilerGeneratedAttribute] set { } }
         protected internal System.Text.Json.JsonSerializerOptions JsonSerializerOptions { [System.Runtime.CompilerServices.CompilerGeneratedAttribute] get { throw null; } }
-        protected abstract void BeginInvokeJS(long taskId, string identifier, string argsJson);
+        protected abstract void BeginInvokeJS(long taskId, string identifier, string? argsJson);
         protected internal abstract void EndInvokeDotNet(Microsoft.JSInterop.Infrastructure.DotNetInvocationInfo invocationInfo, in Microsoft.JSInterop.Infrastructure.DotNetInvocationResult invocationResult);
+        [System.Diagnostics.DebuggerStepThroughAttribute]
         public System.Threading.Tasks.ValueTask<TValue> InvokeAsync<TValue>(string identifier, object[] args) { throw null; }
         public System.Threading.Tasks.ValueTask<TValue> InvokeAsync<TValue>(string identifier, System.Threading.CancellationToken cancellationToken, object[] args) { throw null; }
     }
@@ -74,7 +75,7 @@ namespace Microsoft.JSInterop.Infrastructure
     {
         public static void BeginInvokeDotNet(Microsoft.JSInterop.JSRuntime jsRuntime, Microsoft.JSInterop.Infrastructure.DotNetInvocationInfo invocationInfo, string argsJson) { }
         public static void EndInvokeJS(Microsoft.JSInterop.JSRuntime jsRuntime, string arguments) { }
-        public static string Invoke(Microsoft.JSInterop.JSRuntime jsRuntime, in Microsoft.JSInterop.Infrastructure.DotNetInvocationInfo invocationInfo, string argsJson) { throw null; }
+        public static string? Invoke(Microsoft.JSInterop.JSRuntime jsRuntime, in Microsoft.JSInterop.Infrastructure.DotNetInvocationInfo invocationInfo, string argsJson) { throw null; }
     }
     [System.Runtime.InteropServices.StructLayoutAttribute(System.Runtime.InteropServices.LayoutKind.Sequential)]
     public readonly partial struct DotNetInvocationInfo
@@ -92,11 +93,11 @@ namespace Microsoft.JSInterop.Infrastructure
     {
         private readonly object _dummy;
         private readonly int _dummyPrimitive;
-        public DotNetInvocationResult(System.Exception exception, string errorKind) { throw null; }
-        public DotNetInvocationResult(object result) { throw null; }
-        public string ErrorKind { [System.Runtime.CompilerServices.CompilerGeneratedAttribute] get { throw null; } }
-        public System.Exception Exception { [System.Runtime.CompilerServices.CompilerGeneratedAttribute] get { throw null; } }
-        public object Result { [System.Runtime.CompilerServices.CompilerGeneratedAttribute] get { throw null; } }
+        public DotNetInvocationResult(System.Exception exception, string? errorKind) { throw null; }
+        public DotNetInvocationResult(object? result) { throw null; }
+        public string? ErrorKind { [System.Runtime.CompilerServices.CompilerGeneratedAttribute] get { throw null; } }
+        public System.Exception? Exception { [System.Runtime.CompilerServices.CompilerGeneratedAttribute] get { throw null; } }
+        public object? Result { [System.Runtime.CompilerServices.CompilerGeneratedAttribute] get { throw null; } }
         public bool Success { [System.Runtime.CompilerServices.CompilerGeneratedAttribute] get { throw null; } }
     }
 }
diff --git a/src/JSInterop/Microsoft.JSInterop/src/DotNetObjectReferenceOfT.cs b/src/JSInterop/Microsoft.JSInterop/src/DotNetObjectReferenceOfT.cs
index 773c2ed9a3..f72a636111 100644
--- a/src/JSInterop/Microsoft.JSInterop/src/DotNetObjectReferenceOfT.cs
+++ b/src/JSInterop/Microsoft.JSInterop/src/DotNetObjectReferenceOfT.cs
@@ -18,7 +18,7 @@ namespace Microsoft.JSInterop
     {
         private readonly TValue _value;
         private long _objectId;
-        private JSRuntime _jsRuntime;
+        private JSRuntime? _jsRuntime;
 
         /// <summary>
         /// Initializes a new instance of <see cref="DotNetObjectReference{TValue}" />.
@@ -57,7 +57,7 @@ namespace Microsoft.JSInterop
             }
         }
 
-        internal JSRuntime JSRuntime
+        internal JSRuntime? JSRuntime
         {
             get
             {
diff --git a/src/JSInterop/Microsoft.JSInterop/src/Infrastructure/DotNetDispatcher.cs b/src/JSInterop/Microsoft.JSInterop/src/Infrastructure/DotNetDispatcher.cs
index 8d37905980..30c1910143 100644
--- a/src/JSInterop/Microsoft.JSInterop/src/Infrastructure/DotNetDispatcher.cs
+++ b/src/JSInterop/Microsoft.JSInterop/src/Infrastructure/DotNetDispatcher.cs
@@ -34,14 +34,14 @@ namespace Microsoft.JSInterop.Infrastructure
         /// <param name="invocationInfo">The <see cref="DotNetInvocationInfo"/>.</param>
         /// <param name="argsJson">A JSON representation of the parameters.</param>
         /// <returns>A JSON representation of the return value, or null.</returns>
-        public static string Invoke(JSRuntime jsRuntime, in DotNetInvocationInfo invocationInfo, string argsJson)
+        public static string? Invoke(JSRuntime jsRuntime, in DotNetInvocationInfo invocationInfo, string argsJson)
         {
             // This method doesn't need [JSInvokable] because the platform is responsible for having
             // some way to dispatch calls here. The logic inside here is the thing that checks whether
             // the targeted method has [JSInvokable]. It is not itself subject to that restriction,
             // because there would be nobody to police that. This method *is* the police.
 
-            IDotNetObjectReference targetInstance = default;
+            IDotNetObjectReference? targetInstance = default;
             if (invocationInfo.DotNetObjectId != default)
             {
                 targetInstance = jsRuntime.GetObjectReference(invocationInfo.DotNetObjectId);
@@ -75,9 +75,9 @@ namespace Microsoft.JSInterop.Infrastructure
 
             var callId = invocationInfo.CallId;
 
-            object syncResult = null;
-            ExceptionDispatchInfo syncException = null;
-            IDotNetObjectReference targetInstance = null;
+            object? syncResult = null;
+            ExceptionDispatchInfo? syncException = null;
+            IDotNetObjectReference? targetInstance = null;
             try
             {
                 if (invocationInfo.DotNetObjectId != default)
@@ -126,7 +126,7 @@ namespace Microsoft.JSInterop.Infrastructure
             }
         }
 
-        private static object InvokeSynchronously(JSRuntime jsRuntime, in DotNetInvocationInfo callInfo, IDotNetObjectReference objectReference, string argsJson)
+        private static object? InvokeSynchronously(JSRuntime jsRuntime, in DotNetInvocationInfo callInfo, IDotNetObjectReference? objectReference, string argsJson)
         {
             var assemblyName = callInfo.AssemblyName;
             var methodIdentifier = callInfo.MethodIdentifier;
@@ -168,14 +168,14 @@ namespace Microsoft.JSInterop.Infrastructure
                 if (tie.InnerException != null)
                 {
                     ExceptionDispatchInfo.Capture(tie.InnerException).Throw();
-                    throw null; // unreached
+                    throw tie.InnerException; // Unreachable
                 }
 
                 throw;
             }
         }
 
-        internal static object[] ParseArguments(JSRuntime jsRuntime, string methodIdentifier, string arguments, Type[] parameterTypes)
+        internal static object?[] ParseArguments(JSRuntime jsRuntime, string methodIdentifier, string arguments, Type[] parameterTypes)
         {
             if (parameterTypes.Length == 0)
             {
@@ -189,7 +189,7 @@ namespace Microsoft.JSInterop.Infrastructure
                 throw new JsonException("Invalid JSON");
             }
 
-            var suppliedArgs = new object[parameterTypes.Length];
+            var suppliedArgs = new object?[parameterTypes.Length];
 
             var index = 0;
             while (index < parameterTypes.Length && reader.Read() && reader.TokenType != JsonTokenType.EndArray)
@@ -331,7 +331,7 @@ namespace Microsoft.JSInterop.Infrastructure
 
                 foreach (var method in invokableMethods)
                 {
-                    var identifier = method.GetCustomAttribute<JSInvokableAttribute>(false).Identifier ?? method.Name;
+                    var identifier = method.GetCustomAttribute<JSInvokableAttribute>(false)!.Identifier ?? method.Name!;
                     var parameterTypes = method.GetParameters().Select(p => p.ParameterType).ToArray();
 
                     if (result.ContainsKey(identifier))
@@ -360,7 +360,7 @@ namespace Microsoft.JSInterop.Infrastructure
                 .Where(method => !method.ContainsGenericParameters && method.IsDefined(typeof(JSInvokableAttribute), inherit: false));
             foreach (var method in invokableMethods)
             {
-                var identifier = method.GetCustomAttribute<JSInvokableAttribute>(false).Identifier ?? method.Name;
+                var identifier = method.GetCustomAttribute<JSInvokableAttribute>(false)!.Identifier ?? method.Name;
                 var parameterTypes = method.GetParameters().Select(p => p.ParameterType).ToArray();
 
                 if (result.ContainsKey(identifier))
@@ -400,7 +400,7 @@ namespace Microsoft.JSInterop.Infrastructure
             public AssemblyKey(Assembly assembly)
             {
                 Assembly = assembly;
-                AssemblyName = assembly.GetName().Name;
+                AssemblyName = assembly.GetName().Name!;
             }
 
             public AssemblyKey(string assemblyName)
@@ -409,7 +409,7 @@ namespace Microsoft.JSInterop.Infrastructure
                 AssemblyName = assemblyName;
             }
 
-            public Assembly Assembly { get; }
+            public Assembly? Assembly { get; }
 
             public string AssemblyName { get; }
 
diff --git a/src/JSInterop/Microsoft.JSInterop/src/Infrastructure/DotNetInvocationResult.cs b/src/JSInterop/Microsoft.JSInterop/src/Infrastructure/DotNetInvocationResult.cs
index d62dd532ee..289ba30e61 100644
--- a/src/JSInterop/Microsoft.JSInterop/src/Infrastructure/DotNetInvocationResult.cs
+++ b/src/JSInterop/Microsoft.JSInterop/src/Infrastructure/DotNetInvocationResult.cs
@@ -12,7 +12,7 @@ namespace Microsoft.JSInterop.Infrastructure
         /// </summary>
         /// <param name="exception">The <see cref="System.Exception"/> that caused the failure.</param>
         /// <param name="errorKind">The error kind.</param>
-        public DotNetInvocationResult(Exception exception, string errorKind)
+        public DotNetInvocationResult(Exception exception, string? errorKind)
         {
             Result = default;
             Exception = exception ?? throw new ArgumentNullException(nameof(exception));
@@ -24,7 +24,7 @@ namespace Microsoft.JSInterop.Infrastructure
         /// Constructor for a successful invocation.
         /// </summary>
         /// <param name="result">The result.</param>
-        public DotNetInvocationResult(object result)
+        public DotNetInvocationResult(object? result)
         {
             Result = result;
             Exception = default;
@@ -35,17 +35,17 @@ namespace Microsoft.JSInterop.Infrastructure
         /// <summary>
         /// Gets the <see cref="System.Exception"/> that caused the failure.
         /// </summary>
-        public Exception Exception { get; }
+        public Exception? Exception { get; }
 
         /// <summary>
         /// Gets the error kind.
         /// </summary>
-        public string ErrorKind { get; }
+        public string? ErrorKind { get; }
 
         /// <summary>
         /// Gets the result of a successful invocation.
         /// </summary>
-        public object Result { get; }
+        public object? Result { get; }
 
         /// <summary>
         /// <see langword="true"/> if the invocation succeeded, otherwise <see langword="false"/>.
diff --git a/src/JSInterop/Microsoft.JSInterop/src/Infrastructure/DotNetObjectReferenceJsonConverterFactory.cs b/src/JSInterop/Microsoft.JSInterop/src/Infrastructure/DotNetObjectReferenceJsonConverterFactory.cs
index 288bfdd090..e06601ca0a 100644
--- a/src/JSInterop/Microsoft.JSInterop/src/Infrastructure/DotNetObjectReferenceJsonConverterFactory.cs
+++ b/src/JSInterop/Microsoft.JSInterop/src/Infrastructure/DotNetObjectReferenceJsonConverterFactory.cs
@@ -27,7 +27,7 @@ namespace Microsoft.JSInterop.Infrastructure
             var instanceType = typeToConvert.GetGenericArguments()[0];
             var converterType = typeof(DotNetObjectReferenceJsonConverter<>).MakeGenericType(instanceType);
 
-            return (JsonConverter)Activator.CreateInstance(converterType, JSRuntime);
+            return (JsonConverter)Activator.CreateInstance(converterType, JSRuntime)!;
         }
     }
 }
diff --git a/src/JSInterop/Microsoft.JSInterop/src/Infrastructure/TaskGenericsUtil.cs b/src/JSInterop/Microsoft.JSInterop/src/Infrastructure/TaskGenericsUtil.cs
index 4e14d50783..747bc13631 100644
--- a/src/JSInterop/Microsoft.JSInterop/src/Infrastructure/TaskGenericsUtil.cs
+++ b/src/JSInterop/Microsoft.JSInterop/src/Infrastructure/TaskGenericsUtil.cs
@@ -16,7 +16,7 @@ namespace Microsoft.JSInterop.Infrastructure
         private static ConcurrentDictionary<Type, ITcsResultSetter> _cachedResultSetters
             = new ConcurrentDictionary<Type, ITcsResultSetter>();
 
-        public static void SetTaskCompletionSourceResult(object taskCompletionSource, object result)
+        public static void SetTaskCompletionSourceResult(object taskCompletionSource, object? result)
             => CreateResultSetter(taskCompletionSource).SetResult(taskCompletionSource, result);
 
         public static void SetTaskCompletionSourceException(object taskCompletionSource, Exception exception)
@@ -25,7 +25,7 @@ namespace Microsoft.JSInterop.Infrastructure
         public static Type GetTaskCompletionSourceResultType(object taskCompletionSource)
             => CreateResultSetter(taskCompletionSource).ResultType;
 
-        public static object GetTaskResult(Task task)
+        public static object? GetTaskResult(Task task)
         {
             var getter = _cachedResultGetters.GetOrAdd(task.GetType(), taskInstanceType =>
             {
@@ -33,12 +33,12 @@ namespace Microsoft.JSInterop.Infrastructure
                 return resultType == null
                     ? new VoidTaskResultGetter()
                     : (ITaskResultGetter)Activator.CreateInstance(
-                        typeof(TaskResultGetter<>).MakeGenericType(resultType));
+                        typeof(TaskResultGetter<>).MakeGenericType(resultType))!;
             });
             return getter.GetResult(task);
         }
 
-        private static Type GetTaskResultType(Type taskType)
+        private static Type? GetTaskResultType(Type taskType)
         {
             // It might be something derived from Task or Task<T>, so we have to scan
             // up the inheritance hierarchy to find the Task or Task<T>
@@ -57,23 +57,23 @@ namespace Microsoft.JSInterop.Infrastructure
         interface ITcsResultSetter
         {
             Type ResultType { get; }
-            void SetResult(object taskCompletionSource, object result);
+            void SetResult(object taskCompletionSource, object? result);
             void SetException(object taskCompletionSource, Exception exception);
         }
 
         private interface ITaskResultGetter
         {
-            object GetResult(Task task);
+            object? GetResult(Task task);
         }
 
         private class TaskResultGetter<T> : ITaskResultGetter
         {
-            public object GetResult(Task task) => ((Task<T>)task).Result;
+            public object? GetResult(Task task) => ((Task<T>)task).Result!;
         }
 
         private class VoidTaskResultGetter : ITaskResultGetter
         {
-            public object GetResult(Task task)
+            public object? GetResult(Task task)
             {
                 task.Wait(); // Throw if the task failed
                 return null;
@@ -84,7 +84,7 @@ namespace Microsoft.JSInterop.Infrastructure
         {
             public Type ResultType => typeof(T);
 
-            public void SetResult(object tcs, object result)
+            public void SetResult(object tcs, object? result)
             {
                 var typedTcs = (TaskCompletionSource<T>)tcs;
 
@@ -93,7 +93,7 @@ namespace Microsoft.JSInterop.Infrastructure
                     ? resultT
                     : (T)Convert.ChangeType(result, typeof(T));
 
-                typedTcs.SetResult(typedResult);
+                typedTcs.SetResult(typedResult!);
             }
 
             public void SetException(object tcs, Exception exception)
@@ -109,7 +109,7 @@ namespace Microsoft.JSInterop.Infrastructure
             {
                 var resultType = tcsType.GetGenericArguments().Single();
                 return (ITcsResultSetter)Activator.CreateInstance(
-                    typeof(TcsResultSetter<>).MakeGenericType(resultType));
+                    typeof(TcsResultSetter<>).MakeGenericType(resultType))!;
             });
         }
     }
diff --git a/src/JSInterop/Microsoft.JSInterop/src/JSInProcessRuntime.cs b/src/JSInterop/Microsoft.JSInterop/src/JSInProcessRuntime.cs
index e84ab1b631..5125cbd701 100644
--- a/src/JSInterop/Microsoft.JSInterop/src/JSInProcessRuntime.cs
+++ b/src/JSInterop/Microsoft.JSInterop/src/JSInProcessRuntime.cs
@@ -1,6 +1,7 @@
 // Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
+using System.Diagnostics.CodeAnalysis;
 using System.Text.Json;
 
 namespace Microsoft.JSInterop
@@ -17,6 +18,7 @@ namespace Microsoft.JSInterop
         /// <param name="identifier">An identifier for the function to invoke. For example, the value <c>"someScope.someFunction"</c> will invoke the function <c>window.someScope.someFunction</c>.</param>
         /// <param name="args">JSON-serializable arguments.</param>
         /// <returns>An instance of <typeparamref name="TValue"/> obtained by JSON-deserializing the return value.</returns>
+        [return: MaybeNull]
         public TValue Invoke<TValue>(string identifier, params object[] args)
         {
             var resultJson = InvokeJS(identifier, JsonSerializer.Serialize(args, JsonSerializerOptions));
@@ -34,6 +36,6 @@ namespace Microsoft.JSInterop
         /// <param name="identifier">The identifier for the function to invoke.</param>
         /// <param name="argsJson">A JSON representation of the arguments.</param>
         /// <returns>A JSON representation of the result.</returns>
-        protected abstract string InvokeJS(string identifier, string argsJson);
+        protected abstract string? InvokeJS(string identifier, string? argsJson);
     }
 }
diff --git a/src/JSInterop/Microsoft.JSInterop/src/JSInvokableAttribute.cs b/src/JSInterop/Microsoft.JSInterop/src/JSInvokableAttribute.cs
index b710d54b2c..9f062af8b8 100644
--- a/src/JSInterop/Microsoft.JSInterop/src/JSInvokableAttribute.cs
+++ b/src/JSInterop/Microsoft.JSInterop/src/JSInvokableAttribute.cs
@@ -20,7 +20,7 @@ namespace Microsoft.JSInterop
         /// If not set, the identifier is taken from the name of the method. In this case the
         /// method name must be unique within the assembly.
         /// </summary>
-        public string Identifier { get; }
+        public string? Identifier { get; }
 
         /// <summary>
         /// Constructs an instance of <see cref="JSInvokableAttribute"/> without setting
diff --git a/src/JSInterop/Microsoft.JSInterop/src/JSRuntime.cs b/src/JSInterop/Microsoft.JSInterop/src/JSRuntime.cs
index 8a97ada859..b59e987686 100644
--- a/src/JSInterop/Microsoft.JSInterop/src/JSRuntime.cs
+++ b/src/JSInterop/Microsoft.JSInterop/src/JSRuntime.cs
@@ -62,14 +62,16 @@ namespace Microsoft.JSInterop
         /// <param name="identifier">An identifier for the function to invoke. For example, the value <c>"someScope.someFunction"</c> will invoke the function <c>window.someScope.someFunction</c>.</param>
         /// <param name="args">JSON-serializable arguments.</param>
         /// <returns>An instance of <typeparamref name="TValue"/> obtained by JSON-deserializing the return value.</returns>
-        public ValueTask<TValue> InvokeAsync<TValue>(string identifier, object[] args)
+        public async ValueTask<TValue> InvokeAsync<TValue>(string identifier, object[] args)
         {
             if (DefaultAsyncTimeout.HasValue)
             {
-                return InvokeWithDefaultCancellation<TValue>(identifier, args);
+                using var cts = new CancellationTokenSource(DefaultAsyncTimeout.Value);
+                // We need to await here due to the using
+                return await InvokeAsync<TValue>(identifier, cts.Token, args);
             }
 
-            return InvokeAsync<TValue>(identifier, CancellationToken.None, args);
+            return await InvokeAsync<TValue>(identifier, CancellationToken.None, args);
         }
 
         /// <summary>
@@ -130,22 +132,13 @@ namespace Microsoft.JSInterop
             }
         }
 
-        private async ValueTask<T> InvokeWithDefaultCancellation<T>(string identifier, object[] args)
-        {
-            using (var cts = new CancellationTokenSource(DefaultAsyncTimeout.Value))
-            {
-                // We need to await here due to the using
-                return await InvokeAsync<T>(identifier, cts.Token, args);
-            }
-        }
-
         /// <summary>
         /// Begins an asynchronous function invocation.
         /// </summary>
         /// <param name="taskId">The identifier for the function invocation, or zero if no async callback is required.</param>
         /// <param name="identifier">The identifier for the function to invoke.</param>
         /// <param name="argsJson">A JSON representation of the arguments.</param>
-        protected abstract void BeginInvokeJS(long taskId, string identifier, string argsJson);
+        protected abstract void BeginInvokeJS(long taskId, string identifier, string? argsJson);
 
         /// <summary>
         /// Completes an async JS interop call from JavaScript to .NET
diff --git a/src/JSInterop/Microsoft.JSInterop/src/Microsoft.JSInterop.csproj b/src/JSInterop/Microsoft.JSInterop/src/Microsoft.JSInterop.csproj
index 2c8e56a9eb..07f461121a 100644
--- a/src/JSInterop/Microsoft.JSInterop/src/Microsoft.JSInterop.csproj
+++ b/src/JSInterop/Microsoft.JSInterop/src/Microsoft.JSInterop.csproj
@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
     <TargetFrameworks>netstandard2.0;$(DefaultNetCoreTargetFramework)</TargetFrameworks>
@@ -8,6 +8,7 @@
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <IsPackable>true</IsPackable>
     <IsAspNetCoreApp>true</IsAspNetCoreApp>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0'">
diff --git a/src/JSInterop/Microsoft.JSInterop/test/Infrastructure/DotNetDispatcherTest.cs b/src/JSInterop/Microsoft.JSInterop/test/Infrastructure/DotNetDispatcherTest.cs
index 1f482b2680..9ffbefe864 100644
--- a/src/JSInterop/Microsoft.JSInterop/test/Infrastructure/DotNetDispatcherTest.cs
+++ b/src/JSInterop/Microsoft.JSInterop/test/Infrastructure/DotNetDispatcherTest.cs
@@ -1,6 +1,7 @@
 // Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
+#nullable disable
 using System;
 using System.Linq;
 using System.Text.Json;
diff --git a/src/JSInterop/Microsoft.JSInterop/test/Infrastructure/DotNetObjectReferenceJsonConverterTest.cs b/src/JSInterop/Microsoft.JSInterop/test/Infrastructure/DotNetObjectReferenceJsonConverterTest.cs
index 8d055aea2c..b10c5aab81 100644
--- a/src/JSInterop/Microsoft.JSInterop/test/Infrastructure/DotNetObjectReferenceJsonConverterTest.cs
+++ b/src/JSInterop/Microsoft.JSInterop/test/Infrastructure/DotNetObjectReferenceJsonConverterTest.cs
@@ -80,7 +80,7 @@ namespace Microsoft.JSInterop.Infrastructure
             var json = $"{{\"__dotNetObject\":{objectId}}}";
 
             // Act
-            var deserialized = JsonSerializer.Deserialize<DotNetObjectReference<TestModel>>(json, JsonSerializerOptions);
+            var deserialized = JsonSerializer.Deserialize<DotNetObjectReference<TestModel>>(json, JsonSerializerOptions)!;
 
             // Assert
             Assert.Same(input, deserialized.Value);
@@ -100,7 +100,7 @@ namespace Microsoft.JSInterop.Infrastructure
             var json = $"[{{\"__dotNetObject\":{JSRuntime.TrackObjectReference(ref1)}}},{{\"__dotNetObject\":{JSRuntime.TrackObjectReference(ref2)}}}]";
 
             // Act
-            var deserialized = JsonSerializer.Deserialize<DotNetObjectReference<TestModel>[]>(json, JsonSerializerOptions);
+            var deserialized = JsonSerializer.Deserialize<DotNetObjectReference<TestModel>[]>(json, JsonSerializerOptions)!;
 
             // Assert
             Assert.Same(instance1, deserialized[0].Value);
@@ -121,7 +121,7 @@ namespace Microsoft.JSInterop.Infrastructure
 }}";
 
             // Act
-            var deserialized = JsonSerializer.Deserialize<DotNetObjectReference<TestModel>>(json, JsonSerializerOptions);
+            var deserialized = JsonSerializer.Deserialize<DotNetObjectReference<TestModel>>(json, JsonSerializerOptions)!;
 
             // Assert
             Assert.Same(input, deserialized.Value);
diff --git a/src/JSInterop/Microsoft.JSInterop/test/JSInProcessRuntimeTest.cs b/src/JSInterop/Microsoft.JSInterop/test/JSInProcessRuntimeTest.cs
index f42e0801a0..7d6f341240 100644
--- a/src/JSInterop/Microsoft.JSInterop/test/JSInProcessRuntimeTest.cs
+++ b/src/JSInterop/Microsoft.JSInterop/test/JSInProcessRuntimeTest.cs
@@ -21,7 +21,7 @@ namespace Microsoft.JSInterop
             };
 
             // Act
-            var syncResult = runtime.Invoke<TestDTO>("test identifier 1", "arg1", 123, true);
+            var syncResult = runtime.Invoke<TestDTO>("test identifier 1", "arg1", 123, true)!;
             var call = runtime.InvokeCalls.Single();
 
             // Assert
@@ -80,7 +80,7 @@ namespace Microsoft.JSInterop
                 "test identifier",
                 DotNetObjectReference.Create(obj1),
                 "some other arg",
-                DotNetObjectReference.Create(obj2));
+                DotNetObjectReference.Create(obj2))!;
             var call = runtime.InvokeCalls.Single();
 
             // Assert
@@ -90,16 +90,16 @@ namespace Microsoft.JSInterop
         class TestDTO
         {
             public int IntValue { get; set; }
-            public string StringValue { get; set; }
+            public string? StringValue { get; set; }
         }
 
         class TestJSInProcessRuntime : JSInProcessRuntime
         {
             public List<InvokeArgs> InvokeCalls { get; set; } = new List<InvokeArgs>();
 
-            public string NextResultJson { get; set; }
+            public string? NextResultJson { get; set; }
 
-            protected override string InvokeJS(string identifier, string argsJson)
+            protected override string? InvokeJS(string identifier, string? argsJson)
             {
                 InvokeCalls.Add(new InvokeArgs { Identifier = identifier, ArgsJson = argsJson });
                 return NextResultJson;
@@ -107,11 +107,11 @@ namespace Microsoft.JSInterop
 
             public class InvokeArgs
             {
-                public string Identifier { get; set; }
-                public string ArgsJson { get; set; }
+                public string? Identifier { get; set; }
+                public string? ArgsJson { get; set; }
             }
 
-            protected override void BeginInvokeJS(long asyncHandle, string identifier, string argsJson)
+            protected override void BeginInvokeJS(long asyncHandle, string identifier, string? argsJson)
                 => throw new NotImplementedException("This test only covers sync calls");
 
             protected internal override void EndInvokeDotNet(DotNetInvocationInfo invocationInfo, in DotNetInvocationResult invocationResult)
diff --git a/src/JSInterop/Microsoft.JSInterop/test/JSRuntimeTest.cs b/src/JSInterop/Microsoft.JSInterop/test/JSRuntimeTest.cs
index 66e0033d2a..7710ba0e8e 100644
--- a/src/JSInterop/Microsoft.JSInterop/test/JSRuntimeTest.cs
+++ b/src/JSInterop/Microsoft.JSInterop/test/JSRuntimeTest.cs
@@ -322,14 +322,14 @@ namespace Microsoft.JSInterop
 
         private class JSError
         {
-            public string Message { get; set; }
+            public string? Message { get; set; }
         }
 
         private class TestPoco
         {
             public int Id { get; set; }
 
-            public string Name { get; set; }
+            public string? Name { get; set; }
         }
 
         class TestJSRuntime : JSRuntime
@@ -348,18 +348,18 @@ namespace Microsoft.JSInterop
             public class BeginInvokeAsyncArgs
             {
                 public long AsyncHandle { get; set; }
-                public string Identifier { get; set; }
-                public string ArgsJson { get; set; }
+                public string? Identifier { get; set; }
+                public string? ArgsJson { get; set; }
             }
 
             public class EndInvokeDotNetArgs
             {
-                public string CallId { get; set; }
+                public string? CallId { get; set; }
                 public bool Success { get; set; }
-                public object ResultOrError { get; set; }
+                public object? ResultOrError { get; set; }
             }
 
-            public Func<DotNetInvocationInfo, object> OnDotNetException { get; set; }
+            public Func<DotNetInvocationInfo, object>? OnDotNetException { get; set; }
 
             protected internal override void EndInvokeDotNet(DotNetInvocationInfo invocationInfo, in DotNetInvocationResult invocationResult)
             {
@@ -377,7 +377,7 @@ namespace Microsoft.JSInterop
                 });
             }
 
-            protected override void BeginInvokeJS(long asyncHandle, string identifier, string argsJson)
+            protected override void BeginInvokeJS(long asyncHandle, string identifier, string? argsJson)
             {
                 BeginInvokeCalls.Add(new BeginInvokeAsyncArgs
                 {
diff --git a/src/JSInterop/Microsoft.JSInterop/test/Microsoft.JSInterop.Tests.csproj b/src/JSInterop/Microsoft.JSInterop/test/Microsoft.JSInterop.Tests.csproj
index ba3a91a94e..41e378682e 100644
--- a/src/JSInterop/Microsoft.JSInterop/test/Microsoft.JSInterop.Tests.csproj
+++ b/src/JSInterop/Microsoft.JSInterop/test/Microsoft.JSInterop.Tests.csproj
@@ -2,6 +2,7 @@
 
   <PropertyGroup>
     <TargetFrameworks>$(DefaultNetCoreTargetFramework);net472</TargetFrameworks>
+    <Nullable>enable</Nullable>
   </PropertyGroup>
 
   <ItemGroup>
diff --git a/src/JSInterop/Microsoft.JSInterop/test/TestJSRuntime.cs b/src/JSInterop/Microsoft.JSInterop/test/TestJSRuntime.cs
index db9c5ddd36..d2e9cc3ba6 100644
--- a/src/JSInterop/Microsoft.JSInterop/test/TestJSRuntime.cs
+++ b/src/JSInterop/Microsoft.JSInterop/test/TestJSRuntime.cs
@@ -8,7 +8,7 @@ namespace Microsoft.JSInterop
 {
     internal class TestJSRuntime : JSRuntime
     {
-        protected override void BeginInvokeJS(long asyncHandle, string identifier, string argsJson)
+        protected override void BeginInvokeJS(long asyncHandle, string identifier, string? argsJson)
         {
             throw new NotImplementedException();
         }