private function setObjKey($objKey)
    {
        if (empty($objKey) && !is_numeric($objKey)) {
            $this->objKey = null;
            return $this;
        }

        if (!is_string($objKey)) {
            throw new InvalidArgumentException(
                'Can not set model loader object type: not a string'
            );
        }

        $this->objKey = $objKey;
        return $this;
    }