diff --git a/src/Microsoft.Blazor.Browser.JS/src/Platform/Mono/MonoPlatform.ts b/src/Microsoft.Blazor.Browser.JS/src/Platform/Mono/MonoPlatform.ts
index 90bb87b8ed..1a37ccd00f 100644
--- a/src/Microsoft.Blazor.Browser.JS/src/Platform/Mono/MonoPlatform.ts
+++ b/src/Microsoft.Blazor.Browser.JS/src/Platform/Mono/MonoPlatform.ts
@@ -86,10 +86,6 @@ export const monoPlatform: Platform = {
     //FIXME this is wastefull, we could remove the temp malloc by going the UTF16 route
     //FIXME this is unsafe, cuz raw objects could be GC'd.
 
-    if (!managedString) {
-      return null;
-    }
-
     const utf8 = mono_string_get_utf8(managedString);
     const res = Module.UTF8ToString(utf8);
     Module._free(utf8 as any);
@@ -141,7 +137,7 @@ function addScriptTagsToDocument() {
   document.write(`<script defer src="${monoRuntimeScriptUrl}"></script>`);
 }
 
-function createEmscriptenModuleInstance(loadAssemblyUrls: string[], onReady: () => void, onError: (reason: any) => void) {
+function createEmscriptenModuleInstance(loadAssemblyUrls: string[], onReady: () => void, onError: (reason?: any) => void) {
   const module = {} as typeof Module;
 
   module.print = line => console.log(`WASM: ${line}`);
@@ -163,7 +159,7 @@ function createEmscriptenModuleInstance(loadAssemblyUrls: string[], onReady: ()
 
     Module.FS_createPath('/', 'appBinDir', true, true);
     loadAssemblyUrls.forEach(url =>
-      FS.createPreloadedFile('appBinDir', `${getAssemblyNameFromUrl(url)}.dll`, url, true, false, null, <any>onError));
+      FS.createPreloadedFile('appBinDir', `${getAssemblyNameFromUrl(url)}.dll`, url, true, false, undefined, onError));
   });
 
   module.postRun.push(() => {
diff --git a/src/Microsoft.Blazor.Browser.JS/src/Platform/Platform.ts b/src/Microsoft.Blazor.Browser.JS/src/Platform/Platform.ts
index 958f023bc5..8f4b610504 100644
--- a/src/Microsoft.Blazor.Browser.JS/src/Platform/Platform.ts
+++ b/src/Microsoft.Blazor.Browser.JS/src/Platform/Platform.ts
@@ -3,7 +3,7 @@
 
   callEntryPoint(assemblyName: string, args: System_Object[]);
   findMethod(assemblyName: string, namespace: string, className: string, methodName: string): MethodHandle;
-  callMethod(method: MethodHandle, target: System_Object, args: System_Object[]): System_Object;
+  callMethod(method: MethodHandle, target: System_Object | null, args: System_Object[]): System_Object;
 
   toJavaScriptString(dotNetString: System_String): string;
   toDotNetString(javaScriptString: string): System_String;
diff --git a/src/Microsoft.Blazor.Browser.JS/src/Rendering/Renderer.ts b/src/Microsoft.Blazor.Browser.JS/src/Rendering/Renderer.ts
index fa4b1fa9e1..911b573fe9 100644
--- a/src/Microsoft.Blazor.Browser.JS/src/Rendering/Renderer.ts
+++ b/src/Microsoft.Blazor.Browser.JS/src/Rendering/Renderer.ts
@@ -173,7 +173,7 @@ function insertText(intoDomElement: Element, textNode: RenderTreeNodePointer) {
 }
 
 function clearElement(element: Element) {
-  let childNode: Node;
+  let childNode: Node | null;
   while (childNode = element.firstChild) {
     element.removeChild(childNode);
   }
diff --git a/src/Microsoft.Blazor.Browser.JS/tsconfig.json b/src/Microsoft.Blazor.Browser.JS/tsconfig.json
index 14d18afd91..9bf855f747 100644
--- a/src/Microsoft.Blazor.Browser.JS/tsconfig.json
+++ b/src/Microsoft.Blazor.Browser.JS/tsconfig.json
@@ -5,7 +5,8 @@
     "removeComments": false,
     "sourceMap": true,
     "target": "es5",
-    "lib": ["es2015", "dom"]
+    "lib": ["es2015", "dom"],
+    "strict": true
   },
   "exclude": [
     "node_modules",