I want to get product price for client (partner) using xml-rpc in php. I have this request:

$msg = new xmlrpcmsg('execute');
$msg->addParam(new xmlrpcval($this->database, "string"));
$msg->addParam(new xmlrpcval($this->uid, "int"));
$msg->addParam(new xmlrpcval($this->password, "string"));
$msg->addParam(new xmlrpcval("product.pricelist", "string"));
$msg->addParam(new xmlrpcval("price_get", "string"));
$msg->addParam(new xmlrpcval($id_val, "array"));
$msg->addParam(new xmlrpcval($prod_id, "int")); //Problem here???
$msg->addParam(new xmlrpcval($quant, "double"));
$resp = $client->send($msg);

Then I get: Error: :dictionary key must be string If I set $prod_id, "string" - I get:

price_get AttributeError: 'NoneType' object has no attribute 'id'

What do I do wrong?