Sale order webservice issus

Hi everybody,

      i am trying to create new sales order no. but i receiving the type message. Element "Key" in namespace "urn:microsoft-dynamics-schemas/page/salesorder" is missing from the message!

actually i like to sale order no manually create. 

this is my c# coding

         can some please correct this code actually i published page 42   

         so2.Salesorder_Service service = new so2.Salesorder_Service();

            service.UseDefaultCredentials = true;

            service.Url = "http://localhost:7047/Test1/WS/CRONUS%20India%20Ltd/Page/Salesorder";            

            so2.Salesorder order = new so2.Salesorder();

            order.No = "pac";                

            service.Update(ref order);

Any help is greatly appreciated