private static function _getXmlFromIpnMessage(Message $ipnMsg)
    {        
        // Try and load the notification data as xml
        $notificationData = $ipnMsg->getMandatoryField("NotificationData");
        
        $dom = new DOMDocument();
        try {
            $dom->loadXML($notificationData);
        } catch (Exception $ex) {
            throw new OffAmazonPaymentsNotifications_InvalidMessageException(
                "Error with ipn message - NotificationData field does not contain xml, " .
                "contents: " . $notificationData
            );
        }
        
        return $dom->documentElement;
    }