Can I overload a webmethod?

Is there some special mechnism necessary to make this work? If I
compile something like this ...

--- START OF CODE ---
   [WebMethod(Description="Call Process", EnableSession=true)]
   public XmlElement MyMethod(string ProcessName) {
     XmlDocument xml = new XmlDocument();

     return xml.DocumentElement;
   }

   [WebMethod(Description="Call Process with input data", EnableSession=true)]
   public XmlElement MyMethod(string ProcessName, XmlDocument UserData) {
     XmlDocument xml = new XmlDocument();

     return xml.DocumentElement;
   }
--- END OF CODE ---
It compiles OK, but I get an exception when I try and access it.