求助.net问题

两个页面
第一个页面ButtonSearchTyped.aspx
<%@ Page Language="C#" %><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><script runat="server">
    public string SearchString
    {
        get 
        {
            return txtSearch.Text; 
        } 
    }
</script><html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title>Button Search Typed</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <asp:Label ID="lblSearch" runat="server" Text="Search:"></asp:Label>
        <asp:TextBox ID="txtSearch" runat="server"></asp:TextBox>
        <asp:Button ID="btnSearch" runat="server" Text="Go!" PostBackUrl="~/ButtonSearchResultsTyped.aspx" />
    </div>
    </form>
</body>
</html>
第二个页面ButtonSearchResultsTyped
<%@ Page Language="C#" %>
<%@ PreviousPageType VirtualPath="~/ButtonSearchTyped.aspx" %><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><script runat="server">    protected void Page_Load(object sender, EventArgs e)
    {
        if (PreviousPage != null)
        {
            lblSearch.Text = string.Format("Search For: {0}", PreviousPage.SearchString); 
        }
    }
</script><html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title>Button Search Results Typed</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <asp:Label ID="lblSearch" runat="server" Text="Label"></asp:Label>
    </div>
    </form>
</body>
</html>
问题是：第二个页面里的
protected void Page_Load(object sender, EventArgs e)
    {
        if (PreviousPage != null)
        {
            lblSearch.Text = string.Format("Search For: {0}", PreviousPage.SearchString); 
        }
它为什么说我的PreviousPage.SearchString没有被定义？？？？？