Update invoice with API fails with "productid does not have a value"

With vTiger 6.0.4, when I try to update an invoice, giving the same object i got from a retrieve, even without modifiying it, it fails with "productid does not have a value" error.

Although invoice object doesn't have a productid field in its root data, adding one fix the issue.

Example :

• params = "sessionName=sessionId&operation=retrieve&id=$idinstance";
• httpc->get(endpointUrl."?".$params);
• $response = $httpc->currentResponse();
• jsonResponse = Zend_JSON::decode(response['body']);
• $retrievedObject = $jsonResponse['result'];

We got our invoice.

Works :

• $retrievedObject['productid']=0;
• objectJson = Zend_JSON::encode(retrievedObject);
• params = array("sessionName"=>sessionId, "operation"=>'update', "element"=>$objectJson);
• httpc->post(endpointUrl, $params, true);
• $response = $httpc->currentResponse();
• jsonResponse = Zend_JSON::decode(response['body']);

Doesn't work :

• objectJson = Zend_JSON::encode(retrievedObject);
• params = array("sessionName"=>sessionId, "operation"=>'update', "element"=>$objectJson);
• httpc->post(endpointUrl, $params, true);
• $response = $httpc->currentResponse();
• jsonResponse = Zend_JSON::decode(response['body']);