diff --git a/nodeunit/index.d.ts b/nodeunit/index.d.ts
index f00764009b..a68d550151 100644
--- a/nodeunit/index.d.ts
+++ b/nodeunit/index.d.ts
@@ -57,7 +57,7 @@ export interface ITestGroup {
     setUp?: (callback: ICallbackFunction) => void;
     /** The tearDown function is run after each test calls test.done() */
     tearDown?: (callback: ICallbackFunction) => void;
-    [property: string]: ITestGroup | ITestBody | ((callback: ICallbackFunction) => void);
+    [property: string]: ITestGroup | ITestBody | ((callback: ICallbackFunction) => void) | undefined;
 }
 
 export interface ICallbackFunction {
diff --git a/nodeunit/tsconfig.json b/nodeunit/tsconfig.json
index c9b631fc55..51552aede0 100644
--- a/nodeunit/tsconfig.json
+++ b/nodeunit/tsconfig.json
@@ -3,7 +3,7 @@
         "module": "commonjs",
         "target": "es6",
         "noImplicitAny": true,
-        "strictNullChecks": false,
+        "strictNullChecks": true,
         "baseUrl": "../",
         "typeRoots": [
             "../"