When developing with the emulator it is important to ensure the version of the Emulator is appropriate for the version of the client libraries you are using. If you are using an old version of the emulator with a more recent version of the client libraries you can end up with problems where the functionality you are requesting of the service has not been implemented in the emulator.
Given that you said the functionality works when you use the online service, but not when you use the emulator I am guessing this is the problem. Download the latest version of the emulator and you should be fine.
Jason