Проблеми с Dispatcher.BeginInvoke

Получавам „Изисква се препратка към обект за нестатичното поле, метод или свойство „System.Windows.Threading.Dispatcher.BeginInvoke(System.Action)“ за този код.

private void ResponseCompleted(IAsyncResult result)
    {
        HttpWebRequest request = result.AsyncState as HttpWebRequest;
        HttpWebResponse response = request.EndGetResponse(result) as HttpWebResponse;

        using (StreamReader sr = new StreamReader(response.GetResponseStream()))
        {
            Dispatcher.BeginInvoke( () => {
                try
                {
                    XDocument resultsXml = XDocument.Load(sr);
                    QueryCompleted(new QueryCompletedEventArgs(resultsXml));
                }
                catch (XmlException e)
                {
                    XDocument errorXml = new XDocument(new XElement("error", e.Message));
                    QueryCompleted(new QueryCompletedEventArgs(errorXml));
                }
            });

        }
    }
}