Sys.Net.WebServiceProxy InvalidOperationException

Hi All,

We are invoking Asp.Net ajax web service from the client side. So the JavaScript functions have calls like:

// The function to alter the server side state object and set the selected node for the case tree.
function SetTreeStateSelectedNode(caseId, url)
{  
    Sample.XYZ.Method(param1, param2, OnMethodReturn);  
}

function OnMethodReturn(result)
{
   var sessionExpiry = CheckForSessionExpiry(result);
   var error = CheckForErrors(result);
  
   ... process result
}

And on the server side in the ".asmx.cs" file:
namespace Sample
[ScriptService]
class XYZ : WebService
{
       [WebMethod(EnableSession = true)]
        public string Method(string param1, string param2)
     & ...