GCM register return empty reg id

Question

I'm trying a push notification application and in a very beginning stage-registration. The following code always give back an empty id in register()...probably I have not much idea on GCM...what's going on?

private class BackgroundRegister extends AsyncTask<Void,Void,String>{

        @Override
        protected String doInBackground(Void... params) {
            // TODO Auto-generated method stub
            String regId = "";

            GoogleCloudMessaging gcm = GoogleCloudMessaging.getInstance(activity);

            try {
                regId = gcm.register(PROJECT_ID);
                Log.d("reg id", regId+"");
                if (regId == null || regId.isEmpty()){
                    return "";
                } else {
                    storeRegistrationId(regId);
                    Log.d("Register", getRegId()+" registered");
                }
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            return regId;
        }

    }