Could not adapt error

# Based on https://twistedmatrix.com/documents/current/web/howto/web-in-60/session-store.html

class MBSession(Session):
  sessionTimeout = 2592000 # 30 days

class IUser(Interface):
  user_id = Attribute("The id from the database representing the current user.")

class User(object):
  implements(IUser)

  def __init__(self, session):
    self.user_id = 0

registerAdapter(User, MBSession, IUser)

class RetrieveGames(Resource):

  baseLogger = None

  def __init__(self):
    self.baseLogger = logging.getLogger('MB-Server.RetrieveGames')
    logger = MBLogAdapter(self.baseLogger, {'file': 'main.py', 'object': 'RetrieveGames', 'method': '__init__'})
    logger.info('Creating new RetrieveGames')

    Resource.__init__(self)

  def render_GET(self, request):
    logger = MBLogAdapter(self.baseLogger, {'file': 'main.py', 'object': 'RetrieveGames', 'method': 'render_GET'})

    logger.debug('request="%s"', str(request))
    #user = request.getUser()
    #logger.debug('user="%s"', str(user))
    session = request.getSession()
    logger.debug('session="%s"', str(session))
    user = IUser(session) # breaks here
    logger.debug('user="%s"', str(user))
    logger.debug('user.user_id="%s"', str(user.user_id))
    ...

#   File "./main.py", line 292, in render_GET
#    user = IUser(session)
#exceptions.TypeError: ('Could not adapt', <twisted.web.server.Session instance at 0x19c2200>, #<InterfaceClass __main__.IUser>)