def preprocess_cif_category(cif, label):
    """
    The mmcif dictionary values are either str or list(), which is a bit
    tricky to work with. This method makes list() of all of them in
    order to parse all of the in the same way.

    Args:
        vif (dict): mmcif category with the parser output.
        label (str): name of the category

    Returns:
        str: Possible error encountered
    """
    if label not in cif:
        return f"Namespace {label} does not exist."

    check_element = list(cif[label].keys())[0]
    values = (
        cif[label]
        if isinstance(cif[label][check_element], list)
        else {k: [v] for k, v in cif[label].items()}
    )
    cif[label] = values

    return None