diff --git a/src/Microsoft.CodeAnalysis.Razor/DefaultTagHelperDescriptorProvider.cs b/src/Microsoft.CodeAnalysis.Razor/DefaultTagHelperDescriptorProvider.cs
index b0d07db459..3b7c410b0d 100644
--- a/src/Microsoft.CodeAnalysis.Razor/DefaultTagHelperDescriptorProvider.cs
+++ b/src/Microsoft.CodeAnalysis.Razor/DefaultTagHelperDescriptorProvider.cs
@@ -31,8 +31,19 @@ namespace Microsoft.CodeAnalysis.Razor
                 return;
             }
 
+            var @interface = compilation.GetTypeByMetadataName(TagHelperTypes.ITagHelper);
+            var @string = compilation.GetSpecialType(SpecialType.System_String);
+            
+            // Ensure ITagHelper and System.String are available. They may be missing or 
+            // errored if we're missing references.
+            if (@interface == null || @interface.TypeKind == TypeKind.Error ||
+                @string == null || @string.TypeKind == TypeKind.Error)
+            {
+                return;
+            }
+
             var types = new List<INamedTypeSymbol>();
-            var visitor = TagHelperTypeVisitor.Create(compilation, types);
+            var visitor = new TagHelperTypeVisitor(@interface, types);
 
             // We always visit the global namespace.
             visitor.Visit(compilation.Assembly.GlobalNamespace);
diff --git a/src/Microsoft.CodeAnalysis.Razor/TagHelperTypeVisitor.cs b/src/Microsoft.CodeAnalysis.Razor/TagHelperTypeVisitor.cs
index 625e8284c1..dfe5a9f0a1 100644
--- a/src/Microsoft.CodeAnalysis.Razor/TagHelperTypeVisitor.cs
+++ b/src/Microsoft.CodeAnalysis.Razor/TagHelperTypeVisitor.cs
@@ -8,14 +8,8 @@ namespace Microsoft.CodeAnalysis.Razor
     // Visits top-level types and finds interface implementations.
     internal class TagHelperTypeVisitor : SymbolVisitor
     {
-        private INamedTypeSymbol _interface;
-        private List<INamedTypeSymbol> _results;
-
-        public static TagHelperTypeVisitor Create(Compilation compilation, List<INamedTypeSymbol> results)
-        {
-            var @interface = compilation.GetTypeByMetadataName(TagHelperTypes.ITagHelper);
-            return new TagHelperTypeVisitor(@interface, results);
-        }
+        private readonly INamedTypeSymbol _interface;
+        private readonly List<INamedTypeSymbol> _results;
 
         public TagHelperTypeVisitor(INamedTypeSymbol @interface, List<INamedTypeSymbol> results)
         {
@@ -41,12 +35,8 @@ namespace Microsoft.CodeAnalysis.Razor
 
         internal bool IsTagHelper(INamedTypeSymbol symbol)
         {
-            if (_interface == null)
-            {
-                return false;
-            }
-
             return
+                symbol.TypeKind != TypeKind.Error &&
                 symbol.DeclaredAccessibility == Accessibility.Public &&
                 !symbol.IsAbstract &&
                 !symbol.IsGenericType &&
diff --git a/test/Microsoft.CodeAnalysis.Razor.Test/DefaultTagHelperDescriptorProviderTest.cs b/test/Microsoft.CodeAnalysis.Razor.Test/DefaultTagHelperDescriptorProviderTest.cs
index b7cbf6a9f6..04883a123a 100644
--- a/test/Microsoft.CodeAnalysis.Razor.Test/DefaultTagHelperDescriptorProviderTest.cs
+++ b/test/Microsoft.CodeAnalysis.Razor.Test/DefaultTagHelperDescriptorProviderTest.cs
@@ -1,9 +1,12 @@
 // 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.IO;
 using System.Linq;
 using System.Reflection;
 using Microsoft.AspNetCore.Razor.Language;
+using Microsoft.AspNetCore.Razor.TagHelpers;
+using Microsoft.CodeAnalysis.CSharp;
 using Xunit;
 
 namespace Microsoft.CodeAnalysis.Razor
@@ -21,6 +24,7 @@ namespace Microsoft.CodeAnalysis.Razor
             var descriptorProvider = new DefaultTagHelperDescriptorProvider();
 
             var context = TagHelperDescriptorProviderContext.Create();
+            context.SetCompilation(compilation);
             context.ExcludeHidden = true;
 
             // Act 
@@ -33,5 +37,93 @@ namespace Microsoft.CodeAnalysis.Razor
             var editorBrowsableDescriptor = context.Results.Where(descriptor => descriptor.GetTypeName() == editorBrowsableTypeName);
             Assert.Empty(editorBrowsableDescriptor);
         }
+
+        [Fact]
+        public void Execute_NoOpsIfCompilationSymbolIsNotSet()
+        {
+            // Arrange
+            var descriptorProvider = new DefaultTagHelperDescriptorProvider();
+
+            var context = TagHelperDescriptorProviderContext.Create();
+
+            // Act 
+            descriptorProvider.Execute(context);
+
+            // Assert
+            Assert.Empty(context.Results);
+        }
+
+        [Fact]
+        public void Execute_NoOpsIfTagHelperInterfaceCannotBeFound()
+        {
+            // Arrange
+            var references = new[]
+            {
+                MetadataReference.CreateFromFile(typeof(string).Assembly.Location),
+            };
+            var compilation = CSharpCompilation.Create("Test", references: references);
+
+            var descriptorProvider = new DefaultTagHelperDescriptorProvider();
+
+            var context = TagHelperDescriptorProviderContext.Create();
+            context.SetCompilation(compilation);
+
+            // Act 
+            descriptorProvider.Execute(context);
+
+            // Assert
+            Assert.Empty(context.Results);
+        }
+
+        [Fact]
+        public void Execute_NoOpsIfStringCannotBeFound()
+        {
+            // Arrange
+            var references = new[]
+            {
+                MetadataReference.CreateFromFile(typeof(ITagHelper).Assembly.Location),
+            };
+            var compilation = CSharpCompilation.Create("Test", references: references);
+
+            var descriptorProvider = new DefaultTagHelperDescriptorProvider();
+
+            var context = TagHelperDescriptorProviderContext.Create();
+            context.SetCompilation(compilation);
+
+            // Act 
+            descriptorProvider.Execute(context);
+
+            // Assert
+            Assert.Empty(context.Results);
+        }
+
+        [Fact]
+        public void Execute_DiscoversTagHelpersFromCompilation()
+        {
+            // Arrange
+            var references = new[]
+            {
+                MetadataReference.CreateFromFile(typeof(string).Assembly.Location),
+                MetadataReference.CreateFromFile(typeof(ITagHelper).Assembly.Location),
+            };
+            var projectDirectory = TestProject.GetProjectDirectory(GetType());
+            var tagHelperContent = File.ReadAllText(Path.Combine(projectDirectory, "TagHelperTypes.cs"));
+            var syntaxTree = CSharpSyntaxTree.ParseText(tagHelperContent);
+            var compilation = CSharpCompilation.Create("Test", references: references, syntaxTrees: new[] { syntaxTree });
+
+            var descriptorProvider = new DefaultTagHelperDescriptorProvider();
+
+            var context = TagHelperDescriptorProviderContext.Create();
+            context.SetCompilation(compilation);
+
+            // Act 
+            descriptorProvider.Execute(context);
+
+            // Assert
+            Assert.Collection(
+                context.Results.OrderBy(r => r.Name),
+                tagHelper => Assert.Equal(typeof(Valid_InheritedTagHelper).FullName, tagHelper.Name),
+                tagHelper => Assert.Equal(typeof(Valid_PlainTagHelper).FullName, tagHelper.Name));
+        }
     }
 }
diff --git a/test/Microsoft.CodeAnalysis.Razor.Test/TagHelperTypeVisitorTest.cs b/test/Microsoft.CodeAnalysis.Razor.Test/TagHelperTypeVisitorTest.cs
index 8f14017c91..88e26016e5 100644
--- a/test/Microsoft.CodeAnalysis.Razor.Test/TagHelperTypeVisitorTest.cs
+++ b/test/Microsoft.CodeAnalysis.Razor.Test/TagHelperTypeVisitorTest.cs
@@ -98,24 +98,4 @@ namespace Microsoft.CodeAnalysis.Razor.Workspaces
             public string Invoke(string foo) => null;
         }
     }
-
-    public abstract class Invalid_AbstractTagHelper : TagHelper
-    {
-    }
-
-    public class Invalid_GenericTagHelper<T> : TagHelper
-    {
-    }
-
-    internal class Invalid_InternalTagHelper : TagHelper
-    {
-    }
-
-    public class Valid_PlainTagHelper : TagHelper
-    {
-    }
-
-    public class Valid_InheritedTagHelper : Valid_PlainTagHelper
-    {
-    }
 }
diff --git a/test/Microsoft.CodeAnalysis.Razor.Test/TagHelperTypes.cs b/test/Microsoft.CodeAnalysis.Razor.Test/TagHelperTypes.cs
new file mode 100644
index 0000000000..fea9cd5ad3
--- /dev/null
+++ b/test/Microsoft.CodeAnalysis.Razor.Test/TagHelperTypes.cs
@@ -0,0 +1,27 @@
+// 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 Microsoft.AspNetCore.Razor.TagHelpers;
+
+namespace Microsoft.CodeAnalysis.Razor
+{
+    public abstract class Invalid_AbstractTagHelper : TagHelper
+    {
+    }
+
+    public class Invalid_GenericTagHelper<T> : TagHelper
+    {
+    }
+
+    internal class Invalid_InternalTagHelper : TagHelper
+    {
+    }
+
+    public class Valid_PlainTagHelper : TagHelper
+    {
+    }
+
+    public class Valid_InheritedTagHelper : Valid_PlainTagHelper
+    {
+    }
+}