diff --git a/knockout/knockout.d.ts b/knockout/knockout.d.ts
index b43997df3..aa6044e9f 100644
--- a/knockout/knockout.d.ts
+++ b/knockout/knockout.d.ts
@@ -5,7 +5,7 @@
 
 
 interface KnockoutSubscribableFunctions<T> {
-	notifySubscribers(valueToWrite: T, event?: string): void;
+	notifySubscribers(valueToWrite?: T, event?: string): void;
 }
 
 interface KnockoutComputedFunctions<T> {