Kalde SOAP request

Jeg skal kalde en webservice via soap, men kan ikke få det til at fungere. Jeg bruger nedenstående kode.

        XmlDocument doc = new XmlDocument();
        doc.Load(Server.MapPath(xmlfile));
        HttpWebRequest req = (HttpWebRequest)WebRequest.Create("URL");
        if (proxy != null) req.Proxy = new WebProxy(proxy, true);
        // if SOAPAction header is required, add it here...
        req.Headers.Add("SOAPAction", "\"\"");

        req.ContentType = "text/xml;charset=\"utf-8\"";
        req.Accept = "text/xml";
        req.Method = "POST";
        Stream stm = req.GetRequestStream();
        doc.Save(stm);
        stm.Close();
        WebResponse resp = req.GetResponse();
        stm = resp.GetResponseStream();
        StreamReader r = new StreamReader(stm);
        // process SOAP return doc here. For now, we'll just send the XML out to the browser ...
        Response.Write(r.ReadToEnd());

Er det helt forkert eller hvad? Jeg får en fejl 500 (Intern serverfejl) tilbage.