def LookupClients(self, keywords):
    """Returns a list of client URNs associated with keywords.

    Args:
      keywords: The list of keywords to search by.

    Returns:
      A list of client URNs.

    Raises:
      ValueError: A string (single keyword) was passed instead of an iterable.
    """
    if isinstance(keywords, string_types):
      raise ValueError(
          "Keywords should be an iterable, not a string (got %s)." % keywords)

    start_time, filtered_keywords = self._AnalyzeKeywords(keywords)

    keyword_map = data_store.REL_DB.ListClientsForKeywords(
        list(map(self._NormalizeKeyword, filtered_keywords)),
        start_time=start_time)

    results = itervalues(keyword_map)
    relevant_set = set(next(results))

    for hits in results:
      relevant_set &= set(hits)

      if not relevant_set:
        return []

    return sorted(relevant_set)