Facebook apprequests dialog error

I am using FB.ui, with apprequests method, in an application on facebook. When I try to send requests to 50 friends I get a 500 internal server error from facebook's function uiserver.php, and the callback function of FB.ui is never called. Do you know what might causing this error and how can I handle it, in order to print a message to the user a开发者_如何学编程bout this error?

Please check out the link https://developers.facebook.com/docs/reference/dialogs/requests/

It has the sample code that works.