private function _validateIndicator($indicator)
    {
        if ($indicator == null) {
            $indicator = ' ';
        } elseif (strlen($indicator) > 1) {
            $errorMessage = File_MARC_Exception::formatError(File_MARC_Exception::$messages[File_MARC_Exception::ERROR_INVALID_INDICATOR], array("tag" => $this->getTag(), "indicator" => $indicator));
            throw new File_MARC_Exception($errorMessage, File_MARC_Exception::ERROR_INVALID_INDICATOR);
        }
        return $indicator;
    }