def config_map_errors(vol_desc):
    """Sanity check a Config Map volume description and return a string
       for the first problem found.  Return None if no problem is
       found.

    """
    required_fields = ['name']
    accepted_fields = ['name', 'items', 'default_mode', 'optional']
    config_map = vol_desc.get('config_map', None)
    if config_map is None:
        return "No config_map specification in config_map volume description"
    for field in required_fields:
        if field not in config_map.keys():
            return "Config map specification missing required '%s'" % field
    for field in config_map.keys():
        if field not in accepted_fields:
            return "Config map specification has unrecognized '%s'" % field
    return None