private function removeChecksum($code)
    {
        $length = strlen($code);
        if ($length == 13 || $length == 10) {
            $code = substr_replace($code, "", -1);
            return $code;
        } elseif ($length == 12 || $length == 9) {
            return $code;
        } else {
            $this->setValid(false);
            $this->addError(self::ERROR_INVALID_LENGTH);
            return $code;
        }
    }