protected function _setOperatorString($operatorString)
    {
        if (!is_string($operatorString) && !($operatorString instanceof Stringable)) {
            throw $this->_createInvalidArgumentException(
                $this->__('Argument is not a string or stringable object'),
                null,
                null,
                $operatorString
            );
        }

        $this->operatorString = $operatorString;
    }