Description:
------------
see example below

Test script:
---------------
<?php

class Demo extends ArrayObject {

}

$obj = new StdClass();

$demo = new Demo;

$demo[] = $obj;

$data = array(
    $demo,
    $obj,
    $obj,
);

var_dump(msgpack_unserialize(msgpack_serialize($data)));

Expected result:
----------------
no error

Actual result:
--------------
Warning: [msgpack] (msgpack_unserialize_map_item) Invalid references value: 6