How to cancel an appointment using Zimbra soap Api?

Question

I am using Zimbra Soap API to schedule appointments in a PHP Symfony application creation and modification go well but cancelling has not.

This is the code :

public function cancelAppointment($object) {
    $i = $object->getapptId();
    $login = $this->zimbraSoapLogin;
    $password = $this->zimbraSoapPassword;
    $api = MailFactory::instance('https://*******/service/soap');

    $account = new AccountSelector(AccountBy::NAME(), $login);
    $api->auth($account, $password);

    $apt = $api->getAppointment(true, true, true, null, $i);

    $msg = new Msg();
    $msg->setSubject("[annulation]" . $object->getNom());

    $a = $api->modifyAppointment( $msg, $i . '-' . $apt->appt->inv->id);
    return($a) ;
}

and This is the error :

Server error: POST https://**********/service/soap resulted in a 500 Server Error

response:

<soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope">
    <soap:Header><context xmlns="urn:zimbra"><change tok (truncated...)