I'm new to Python. I can't understand why a variable is None at a certain point in my code:

class UsersInRoom(webapp.RequestHandler):
    def get(self):
        room_id = self.request.get("room_id")
        username = self.request.get("username")
        UserInRoom_entities = UserInRoom.gql("WHERE room = :1", room_id).get()
        if UserInRoom_entities:
            for user_in_room in UserInRoom_entities:
                if user_in_room.username == username:
                    user_in_room.put() # last_poll auto updates to now whenenever user_in_room is saved
        else:
            user_in_room = UserInRoom()
            user_in_room.username = username
            user_in_room.put()

            UserInRoom_entities = []
            UserInRoom_entities.append(user_in_room) // error here

        # name is `user_at_room` intead of `user_in_room` to avoid confusion    
        usernames = [user_at_room.username for user_at_room in UserInRoom_entities]
        self.response.out.write(json.dumps(usernames))

The error is:

Traceback (most recent call last):
  File "C:\Program Files\Google\google_appengine\google\appengine\ext\webapp\__init__.py", line 507, in __call__
    handler.get(*groups)
  File "path\to\chat.py", line 160, in get
AttributeError: 'NoneType' object has no attribute 'append'

How is this possible? I'm setting UserInRoom_entities = [] immediately before that call. Or is something else the None in question?