An error occurred while processing this request

Customer customer = (context.Execute<Customer>(new Uri("GetAuthenticatedCustomer", UriKind.Relative), "POST", false, opParams)).FirstOrDefault();

private void loginButton_Click(object sender, RoutedEventArgs e)
        {
            if (!string.IsNullOrEmpty(emailAddressTextbox.Text) && !string.IsNullOrEmpty(passwordTextbox.Text))
            {
                Uri svcUri = new Uri("http://localhost:29875/iShop.svc");

                iShopEntities context = new iShopEntities(svcUri);

                UriOperationParameter[] opParams = new UriOperationParameter[2];

                opParams[0] = new UriOperationParameter("EmailAddress", emailAddressTextbox.Text);
                opParams[1] = new UriOperationParameter("Password", passwordTextbox.Text);

                Customer customer = (context.Execute<Customer>(new Uri("GetAuthenticatedCustomer", UriKind.Relative), "POST", false, opParams)).FirstOrDefault();
                Seller seller = (context.Execute<Seller>(new Uri("GetAuthenticatedSeller", UriKind.Relative), "POST", false, opParams)).FirstOrDefault();

                if (customer != null)
                {
                    MessageBox.Show("Welcome " + customer.FirstName);
                    //MainWindow mainWindow = new MainWindow();
                    //mainWindow.Show();

                    //CustomerOrder customerOrder = new CustomerOrder();
                    //customerOrder.customer = customer;
                    //customerOrder.Show();

                }
                else if (seller != null)
                {
                    MessageBox.Show("Welcome " + seller.FirstName);
                }
            }
            else
            {
                MessageBox.Show("Please specify email and password");
            }
        }