ajax xmlHttpRequest is undefined

ajax xmlHttpRequest is undefined

Hi
I'm having a problem  getting the error status when a session expires on the server

when i receive an http status of 401  the
error function alert returns  xmlHttpRequest = undefined , textStatus = "error" & errorThrown = undefined


how do i get the status of 401 returned to my application
ajaxError & statusCode don't seem to work as well

what am i missing

my ajax  code is as follows

 jQuery.ajax ( {

                  type: 'POST',
                  dataType: "html",
                  contentType: "application/x-www-form-urlencoded; charset=utf-8",
                  url: url,
                  async: false,
                  data: {
                          'objid' : '101'
                        },

                  success: function(msg)
                           {
                             alert ( 'Success' );
                           },
                   error: function ( XMLHttpRequest, textStatus, errorThrown )
                          {
                            alert ( 'Error :' + XMLHttpRequest + ':' + textStatus + ':' + errorThrown );
                          }
                }
            );

the data comming back from the server is
HTTP/1.1 401 Unauthorized
Server: Apache-Coyote/1.1
X-Powered-By: Servlet 2.4; JBoss-4.2.3.GA (build: SVNTag=JBoss_4_2_3_GA date=200807181417)/JBossWeb-2.0
Content-Type: application/json;charset=UTF-8
Content-Length: 18
Date: Thu, 17 Mar 2011 21:26:38 GMT

"Session Expired."