'Check that filter_horizontal is a sequence of field names.'

def _check_filter_horizontal(self, obj):

    if (not isinstance(obj.filter_horizontal, (list, tuple))):
        return must_be('a    list    or    tuple', option='filter_horizontal', obj=obj, id='admin.E018')
    else:
        return list(chain.from_iterable((self._check_filter_item(obj, obj.model, field_name, ('filter_horizontal[%d]' % index)) for (index, field_name) in enumerate(obj.filter_horizontal))))
