Silverlight4:运行返回字符串的存储过程

在Silverlight 4/RIA中执行存储过程时遇到问题。我得到的唯一值是null。我是不是把客户端和服务器端的代码弄错了?

客户端:

public ZipCodesDomainContext _ZipcodesDomainContext = new ZipCodesDomainContext();

        /// <summary>
        /// Creates a new <see cref="MainPage"/> instance.
        /// </summary>
        public MainPage()
        {
            InitializeComponent();
            this.loginContainer.Child = new LoginStatus();
            LoadCity();
        }
private void LoadCity()
        {
            txtCity.Text = _ZipcodesDomainContext.GetCityByZip(42071).Value;
        }

域服务:

public string GetCityByZip(int pZip)
        {
            return ObjectContext.sp_GetCityByZip(pZip).ToString();
        }

数据模型(选择存储过程时):

USE [ZIPCODES]
GO
/****** Object:  StoredProcedure [dbo].[sp_GetCityByZip]    Script Date: 08/23/2010 13:48:11 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
-- =============================================
-- Author:      <Author,,Name>
-- Create date: <Create Date,,>
-- Description: <Description,,>
-- =============================================
ALTER PROCEDURE [dbo].[sp_GetCityByZip] 
    @ZIP int

AS
BEGIN
    -- SET NOCOUNT ON added to prevent extra result sets from
    -- interfering with SELECT statements.
    SET NOCOUNT ON;

    -- Insert statements for procedure here
    SELECT City FROM ZipCodes WHERE Zip = @ZIP
END