xml node xpath stays empty

Hi all, I want to fill my xml:
 
    private string _xmlString =
  @"<?xml version=""1.0"" encoding=""utf-8"" ?>
   <message>
   <content>    
    </content>
    </message>";
    // This method returns an XmlDocument type.
    [WebMethod]
    [ScriptMethod(ResponseFormat = ResponseFormat.Xml)]
    public XmlDocument GetXmlDocument()
    {
        XmlDocument xmlDoc = new XmlDocument();
        xmlDoc.LoadXml(_xmlString);
        XmlNode root = xmlDoc.DocumentElement;
        XmlNode nood;
   ...