find_credentials expects a proxy but gets a deferred (windows)

This happens only on windows:

Traceback (most recent call last):
  File "E:\ubuntuone-control-panel\run-windows-run\ubuntuone\controlpanel\backend.py", line 266, in account_info
    account_info = yield self.wc.call_api(ACCOUNT_API)
  File "E:\ubuntuone-control-panel\run-windows-run\ubuntuone\controlpanel\web_client\txwebclient.py", line 76, in call_api
    d = self.get_credentials()
  File "E:\ubuntuone-control-panel\run-windows-run\ubuntuone\controlpanel\login_client.py", line 34, in get_credentials
    return proxy.find_credentials()
  File "c:\python27\lib\site-packages\twisted\internet\defer.py", line 1141, in unwindGenerator
    return _inlineCallbacks(None, f(*args, **kwargs), Deferred())
--- <exception caught here> ---
  File "c:\python27\lib\site-packages\twisted\internet\defer.py", line 1020, in _inlineCallbacks
    result = g.send(result)
  File "e:\ubuntuone-client\trunk\ubuntuone\credentials.py", line 96, in find_credentials
    sig = proxy.register_to_credentials_found(d.callback)
exceptions.AttributeError: Deferred instance has no attribute 'register_to_credentials_found'