'Validate that filter_vertical is a sequence of field names.'

def validate_filter_vertical(self, cls, model):

    if hasattr(cls, 'filter_vertical'):
        check_isseq(cls, 'filter_vertical', cls.filter_vertical)
        for (idx, field) in enumerate(cls.filter_vertical):
            f = get_field(cls, model, 'filter_vertical', field)
            if (not isinstance(f, models.ManyToManyField)):
                raise ImproperlyConfigured(("'%s.filter_vertical[%d]'    must    be    a    ManyToManyField." % (cls.__name__, idx)))
