Client side web service

Hi, I havewebservice added in Scripmanager Scriptreference tag

  <asp:ScriptManager ID="ScriptManager1" runat="server">
          <Services>
            <asp:ServiceReference Path="~/MyService.asmx" />
        </Services>
    </asp:ScriptManager>

And used the methods of webservice at client side.

I have usedthe following in web.config file to open the application in multiple tabs with different session id:

 <sessionState mode="InProc" timeout="20" cookieless="UseUri">
    </sessionState>

Problem:

I call my page i get error webservice "InvalidException" Object reference not set to the instance of the object.

Please anyone can help..would be greatly appreciated..Thanks :(