diff --git a/src/lib/es6.d.ts b/src/lib/es6.d.ts
index 1d52ef7c8f0..40984ab1512 100644
--- a/src/lib/es6.d.ts
+++ b/src/lib/es6.d.ts
@@ -127,7 +127,7 @@ interface ObjectConstructor {
       * @param target The target object to copy to.
       * @param source The source object from which to copy properties.
       */
-    assign<T, U>(target: T, source: U | null | undefined): T & U;
+    assign<T, U>(target: T, source: U): T & U;
 
     /**
       * Copy the values of all of the enumerable own properties from one or more source objects to a 
@@ -136,7 +136,7 @@ interface ObjectConstructor {
       * @param source1 The first source object from which to copy properties.
       * @param source2 The second source object from which to copy properties.
       */
-    assign<T, U, V>(target: T, source1: U | null | undefined, source2: V | null | undefined): T & U & V;
+    assign<T, U, V>(target: T, source1: U, source2: V): T & U & V;
 
     /**
       * Copy the values of all of the enumerable own properties from one or more source objects to a 
@@ -146,7 +146,7 @@ interface ObjectConstructor {
       * @param source2 The second source object from which to copy properties.
       * @param source3 The third source object from which to copy properties.
       */
-    assign<T, U, V, W>(target: T, source1: U | null | undefined, source2: V | null | undefined, source3: W | null | undefined): T & U & V & W;
+    assign<T, U, V, W>(target: T, source1: U, source2: V, source3: W): T & U & V & W;
 
     /**
       * Copy the values of all of the enumerable own properties from one or more source objects to a