Phalcon 2.0 - update() on relationship model not working.

I just upgrade my project to Phalcon 2.0 yesterday, and my source got geting an error:

Phalcon\Mvc\Model\Message Object
(
    [_type:protected] => InvalidUpdateAttempt
    [_message:protected] => Record cannot be updated because it does not exist
    [_field:protected] => 
    [_model:protected] => 
)

And here is my code:

$invoice = Invoices::findFirst([
    'conditions' => 'display_id = :id:',
    'bind'       => ['id' => $params['invoice_id']]
]);

$invoice->transaction_id = $paypalResponse['PAYMENTINFO_0_TRANSACTIONID'];
$invoice->paid_at        = date('Y-m-d H:i:s');
$invoice->payer          = $tranDetails->PayerInfo->Payer;
$invoice->payer_status   = $tranDetails->PayerInfo->PayerStatus;
$invoice->status = 'Paid';
$invoice->user->increaseBalance($invoice->credit);

$invoice->update();

foreach ($invoice->getMessages() as $msg) {
   print_r('<pre>');
   print_r($msg);
   print_r('</pre>');
}

When I change to use save(); it working but I don't know why update() working on 1.3.4 but not on 2.0. Some people have same issue: https://github.com/phalcon/cphalcon/issues/2981