diff --git a/src/lib/es2015.collection.d.ts b/src/lib/es2015.collection.d.ts
index 6a2c43cd050f7..75a11b5eb17ae 100644
--- a/src/lib/es2015.collection.d.ts
+++ b/src/lib/es2015.collection.d.ts
@@ -4,7 +4,7 @@ interface Map<K, V> {
     forEach(callbackfn: (value: V, key: K, map: Map<K, V>) => void, thisArg?: any): void;
     get(key: K): V | undefined;
     has(key: K): boolean;
-    set(key: K, value?: V): this;
+    set(key: K, value: V): this;
     readonly size: number;
 }
 
@@ -26,7 +26,7 @@ interface WeakMap<K, V> {
     delete(key: K): boolean;
     get(key: K): V | undefined;
     has(key: K): boolean;
-    set(key: K, value?: V): this;
+    set(key: K, value: V): this;
 }
 
 interface WeakMapConstructor {