def read_user_header(head):
    """
    The 'User-Id' header contains the current online user's id, which is an integer
    -1 if no user online
    :param head: the request's header
    :return: online user's id, abort type error if the header is not an integer.
    """
    try:
        return int(head['User-Id'])
    except TypeError:
        print('user_id header of wrong variable type')