diff --git a/lib/lib.es2015.collection.d.ts b/lib/lib.es2015.collection.d.ts
index f014185d9e1..175f4f8725e 100644
--- a/lib/lib.es2015.collection.d.ts
+++ b/lib/lib.es2015.collection.d.ts
@@ -20,7 +20,7 @@ interface Map<K, V> {
     forEach(callbackfn: (value: V, index: 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;
 }
 
@@ -36,7 +36,7 @@ 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 {