Line 101 :

    public function postSerialize(Doctrine_Event $event)
    {
        foreach ($this->_listeners as $listener) {
            $listener->preSerialize($event);
        }
    }

It should be :

    public function postSerialize(Doctrine_Event $event)
    {
        foreach ($this->_listeners as $listener) {
            $listener->postSerialize($event);
        }
    }