为何cookie删除不掉

为什么cookie删除不掉？
点击button1把cookie的Expires设置为以前的日期，但好像不起作用。

C# code

public partial class Result : System.Web.UI.Page{    protected void Page_Load(object sender, EventArgs e)    {        if (Request.Cookies["MyCookie"]!=null)        {            Label1.Text = Request.Cookies["MyCookie"].Value;        }        else        {            Label1.Text = "No cookies";        }    }    protected void Button1_Click(object sender, EventArgs e)    {        if (Request.Cookies["MyCookie"]!=null)        {            Request.Cookies["MyCookie"].Expires = DateTime.Now.AddDays(-1);            Response.Redirect("Result.aspx");        }    }}