为什么XDocument.Parse抛出NotSupportedException

我正在尝试使用XDocument.parse wchich抛出NotSupportedException，就像主题中一样：XDocument.Sarse在WindowsPhone7中不同吗？我根据发布的建议更新了代码，但仍然无济于事。不久前，我使用类似（但更简单）的方法解析RSS，效果很好。

public void sList()
        {
            WebClient client = new WebClient();
            client.Encoding = Encoding.UTF8;
            string url = "http://eztv.it";
            Uri u = new Uri(url);
            client.DownloadStringAsync(u);
            client.DownloadStringCompleted += new DownloadStringCompletedEventHandler(client_DownloadStringCompleted);

        }
    private void client_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
    {
        try
        {
            string s = e.Result;
            s = cut(s);
            XmlReaderSettings settings = new XmlReaderSettings();
            settings.DtdProcessing = DtdProcessing.Ignore;

            XDocument document = null;// XDocument.Parse(s);//Load(s);
            using (XmlReader reader = XmlReader.Create(new StringReader(e.Result), settings))
            {
                document = XDocument.Load(reader); // error thrown here
            }
            // ... rest of code
        }
        catch (Exception ex)
        {
            MessageBox.Show( ex.Message);
        }
    }
    string cut(string s)
    {
        int iod = s.IndexOf("<select name='"SearchString'">");
        int ido = s.LastIndexOf("</select>");
        s = s.Substring(iod, ido - iod + 9);
        return s;
    }

当我用字符串s代替时

//string s = "<select name='"SearchString'"><option value='"308'">10 Things I Hate About You</option><option value='"539'">2 Broke Girls</option></select>";

一切正常，没有任何异常，那么我做错了什么呢？