Google Cloud Endpoints EOFException

I have the method below in AppEngine:

@ApiMethod(name = "authed", path = "greeting/authed")
public HelloGreeting authedGreeting(User user) {
    ...
}

My doInBackground method in Android AsyncTask:

HelloGreeting hg = null;
try {
    hg = service.authed().execute();
} catch (IOException e) {
    Log.d("error", e.getMessage(), e);
}
return hg;

I encountered the ff error:

/_ah/api/.../v1/greeting/authed: java.io.EOFException

In logcat:

Problem accessing /_ah/api/.../v1/greeting/authed. Reason: INTERNAL_SERVER_ERROR
    at java.util.zip.GZIPInputStream.readUByte
    at java.util.zip.GZIPInputStream.readUShort
    at java.util.zip.GZIPInputStream.readUShort

It only work when calling non-auth method. How to fix it?

Im using the local server.