Can't register Handler threads looper as idling resource

I have just found the

  Espresso.registerLooperAsIdlingResource(looper)

method and it seems to be exactly what I need for my tests but unfortunately it is not working. Could someone please explain why calling this code does not make Esspresso wait for Handler thread MessageQueue to become empty. Here is the code example:

 StuffProcessor processor = new StuffProcessor();
 Espresso.registerLooperAsIdlingResource(processor.getLooper());