cache的问题

Question

我在全局中的代码：<script runat="server">
    void Application_Start(object sender, EventArgs e) 
    {
        //在应用程序启动时运行的代码
        OptCache.Add("ysz", "杨思照");
    }
...自已定义的类：public class OptCache
{
public OptCache()
{
//
//TODO: 在此处添加构造函数逻辑
//
}
    public static void Add(string key, string val)
    {
        CacheItemRemovedCallback callBack = new CacheItemRemovedCallback(OnRemove);
        HttpContext.Current.Cache.Insert(
            key,
            val,
            null,
            System.DateTime.Now.AddSeconds(5),
            System.Web.Caching.Cache.NoSlidingExpiration,
            System.Web.Caching.CacheItemPriority.NotRemovable,
            callBack
            );
    }
    public static void OnRemove(string key, object val, CacheItemRemovedReason rea)
    {
        if (HttpContext.Current.Cache[key] != null)
        {
            HttpContext.Current.Cache.Remove(key);
        }
        HttpContext.Current.Cache.Add("ysz",
            "彭小岗",
            null,
            System.Web.Caching.Cache.NoAbsoluteExpiration,
            System.Web.Caching.Cache.NoSlidingExpiration,
            System.Web.Caching.CacheItemPriority.NotRemovable,
            new CacheItemRemovedCallback(OnRemove)
            );
    }
}
页面中：protected void Page_Load(object sender, EventArgs e)
    {
        Response.Write(Cache["ysz"].ToString());
    }
页面在5秒钟之后，点击刷新会报错？？？请大师指点，该如何写，我只是想实现定时更新CACHE！！！