def check_for_empty_nests_in_nest_spec(nest_spec):
    """
    Ensures that the values of `nest_spec` are not empty lists.
    Raises a helpful ValueError if they are.

    Parameters
    ----------
    nest_spec : OrderedDict, or None, optional.
        Keys are strings that define the name of the nests. Values are lists of
        alternative ids, denoting which alternatives belong to which nests.
        Each alternative id must only be associated with a single nest!
        Default == None.

    Returns
    -------
    None.
    """
    empty_nests = []
    for k in nest_spec:
        if len(nest_spec[k]) == 0:
            empty_nests.append(k)
    if empty_nests != []:
        msg = 'The following nests are INCORRECTLY empty: {}'
        raise ValueError(msg.format(empty_nests))
    return None
