diff --git a/typescript/core/ob.ts b/typescript/core/ob.ts
index 50a80b8d60..9c4e4b9472 100644
--- a/typescript/core/ob.ts
+++ b/typescript/core/ob.ts
@@ -71,7 +71,7 @@ export declare class OB {
 
     once(eventName: string, fn: Function): void;
 
-    un(eName: string, fn: Function): void;
+    un(eName: string, fn?: Function): void;
 
     _initRef(): void;