FB.Apprequest problem

 FB.AppRequest("I just smashed you 100  times! Can you beat it?", userFids,"",null,null,"Lets See Data","Friend Smash Challenge!",sendCallBack );  

  void sendCallBack(FBResult result ){
         var jason = Facebook.MiniJSON.Json.Deserialize (result.Text);
         Dictionary<string,object> jdict = jason as Dictionary<string,object>;
         foreach (string key in jdict.Keys) {
             NGUIDebug.Log(" Keys :: "+ key);        
         }

getting error invalid arguments.. i CAnt find what is wrong. I tried to default(int?) in palce of null in #5 argument. Plz help