:param sort_by: string
        :raises: pybomb.exceptions.InvalidSortFieldException

def _validate_sort_field(self, sort_by):
        """
        :param sort_by: string
        :raises: pybomb.exceptions.InvalidSortFieldException
        """
        if (
            sort_by not in self.RESPONSE_FIELD_MAP
            or not self.RESPONSE_FIELD_MAP[sort_by].is_sort
        ):
            raise InvalidSortFieldException(
                '"{0}" is an invalid sort field'.format(sort_by)
            )