android - com.google.api.client.googleapis.json.GoogleJsonResponseException: 500 Internal Server Error -

i getting 500 internal server error while calling app engine endpoint android client. in code looks fine , confused error.

calling endpoint api:

try {         return myapiservice.updateuseraction(userid1, userid2, action).execute().getdata();     } catch (ioexception e) {         return e.getmessage();     } 

useractionendpoint (simplest form):

@apimethod(name = "updateuseraction")     public defaultstringbean updateuseraction(@named("firstuserid") string firstuserid,                                               @named("seconduserid") string seconduserid,                                               @named("action") string action) {          defaultstringbean response = new defaultstringbean();         response.setdata("200");         return response;     } 

and defaultstringbean:

public class defaultstringbean {      private string result;      public string getdata() {         return result;     }      public void setdata(string response) {         result = response;     } } 

why should error?

com.google.api.client.googleapis.json.googlejsonresponseexception: 500 internal server error 

more info: have implemented more endpoints return type of data , none of them thrown exception.