'`fields` should be an item of `fields` or an item of
fieldset[1][\'fields\'] for any `fieldset` in `fieldsets`. It should be a
field name or a tuple of field names.'

def _check_field_spec(self, obj, model, fields, label):

    if isinstance(fields, tuple):
        return list(chain.from_iterable((self._check_field_spec_item(obj, model, field_name, ('%s[%d]' % (label, index))) for (index, field_name) in enumerate(fields))))
    else:
        return self._check_field_spec_item(obj, model, fields, label)
