CRM 2011 - Value cannot be null.\r\nParameter name: detail

Trying to create a simple winform app with below code... when I come to saving am getting an error "{"Value cannot be null.\r\nParameter name: detail"}"

Anyone knows what am doing wrong, something missing ???

System.ServiceModel.Description.ClientCredentials cre = new System.ServiceModel.Description.ClientCredentials();            cre.UserName.UserName = "WWW\\Administrator";            cre.UserName.Password = "ab.1234";                         _serviceProxy = new OrganizationServiceProxy(new Uri("http://win-bvjq2ghxbvn/XRMServices/2011/Discovery.svc"),                                                                    null,                                                                    cre,                                                                   GetDeviceCredentials());                                            _serviceProxy.ServiceConfiguration.CurrentServiceEndpoint.Behaviors.Add(new ProxyTypesBehavior());            IOrganizationService service = (IOrganizationService)_serviceProxy;                        Entity contact = new Entity("contact");            contact["lastname"] = "contact created using IOrganizationSerivce";                        service.Create(contact);