public function readReference()
    {
        $key = $this->_stream->readInt();
        if (!array_key_exists($key, $this->_reference)) {
            require_once 'Zend/Amf/Exception.php';
            throw new Zend_Amf_Exception('Invalid reference key: '. $key);
        }
        return $this->_reference[$key];
    }