private function removeProductCode($code)
    {
        $first3 = substr($code, 0, 3);

        // For ISBN-10, product code is always 978
        if (strlen($code) == 9) {
            $this->setProduct(978);
        }

        // ISBN-13: check that product code is 978 or 979
        elseif ($first3 == 978 || $first3 == 979) {
            $this->setProduct($first3);
            $code = substr($code, 3);
        }

        // Product code is Invalid
        else {
            $this->setValid(false);
            $this->addError(self::ERROR_INVALID_PRODUCT_CODE);
        }

        return $code;
    }