diff --git a/typescript/core/ob.ts b/typescript/core/ob.ts
index 893280633..50a80b8d6 100644
--- a/typescript/core/ob.ts
+++ b/typescript/core/ob.ts
@@ -23,7 +23,7 @@ export interface _OB {
 
     once(eventName: string, fn: Function): void;
 
-    un(eName: string, fn: Function): void;
+    un(eName: string, fn?: Function): void;
 
     _initRef(): void;