diff --git a/typescript/core/ob.ts b/typescript/core/ob.ts
index 893280633..54b10f8ba 100644
--- a/typescript/core/ob.ts
+++ b/typescript/core/ob.ts
@@ -21,7 +21,7 @@ export interface _OB {
 
     on(eventName: string, fn: Function): Function;
 
-    once(eventName: string, fn: Function): void;
+    once(eventName: string, fn?: Function): void;
 
     un(eName: string, fn: Function): void;