diff --git a/mixpanel/mixpanel.d.ts b/mixpanel/mixpanel.d.ts
index 8c84206588..8db3b34a49 100644
--- a/mixpanel/mixpanel.d.ts
+++ b/mixpanel/mixpanel.d.ts
@@ -26,7 +26,7 @@ interface Mixpanel
 
     unregister(propertyName:string):void;
 
-    identify(id:string):void;
+    identify(id?:string):void;
 
     get_distinct_id():string;