flex blazeds response null for first time service call

I am using Flash Builder 4.5 with Blazeds service to develop flex appliacation, here i used java for services and here i am getting empty value from server side for first time service call, after that from next service calling it gives correct result to me, I don't know why i am getting null value for first time hitting the service. Here for the first time hitting java service, the functionality is running at java side but return value is null.

Please any one help me out.

If we use LastResult for service response then I got this error, I used requestHandler for handling response and i got my response for first time itself.

Service.addEventListener("result", resultHandler);


private function resultHandler(evt:ResultEvent):void {
            var ac = evt.result;
          }