android - Why InstanceID always returns a token despite of wrong senderId?

Question

I'm using Google Cloud Messaging api to implement an Android client. To get a token, I do:

   InstanceID ex = InstanceID.getInstance(this.getApplicationContext());
   String regId = ex.getToken(senderId, "GCM", null);

And regId always contains a token, despite of I put on senderId. If I set senderId with "123", InstanceID returns a token! (But the, I don't receive notifications with this token). Why I always get a token? I expect an exception, or a null value maybe...