def get_publishers_of_articles_missing_type(articles):
    """
    :param articles PyMongo collection of articles
    :return: Set: set of publishers of articles that are missing the doc_type
    property, defined as not having the field at all, or having it equal to the
    empty string.
    """
    return articles.distinct(
        "publisher", {
            "$or": [{"doc_type": {"$exists": False}}, {"doc_type": ""}]
        }
    )