public function creditInvoice($invNo, $credNo = "")
    {
        $this->_checkInvNo($invNo);
        $this->_checkCredNo($credNo);

        $digestSecret = self::digest(
            $this->colon($this->_eid, $invNo, $this->_secret)
        );
        $paramList = array(
            $this->_eid,
            $invNo,
            $credNo,
            $digestSecret
        );

        self::printDebug('credit_invoice', $paramList);

        return $this->xmlrpc_call('credit_invoice', $paramList);
    }