SlideShowExtender控件的效果出不来，请高手帮忙！

页面代码<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="SlideShowExtenderTest.aspx.cs" Inherits="LinqTest.SlideShowExtenderTest" %><%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title></title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <asp:ScriptManager ID="ScriptManager1" runat="server">
        </asp:ScriptManager>
        <asp:Panel ID="Panel1" runat="server" Width="300px" HorizontalAlign=Center>
        
        <cc1:SlideShowExtender ID="SlideShowExtender1" runat="server"
         ImageTitleLabelID="Label1" TargetControlID="Image1" UseContextKey="true" 
         PreviousButtonID="Button1" NextButtonID="Button2" PlayButtonID="Button3" 
         SlideShowServiceMethod="GetSlides" ImageDescriptionLabelID="Label2" 
         >
        </cc1:SlideShowExtender>
        <asp:Label ID="Label1" runat="server" Text=""></asp:Label>
        <br />
        <asp:Image ID="Image1" runat="server" Height="331px" ImageUrl="~/ls.jpg" />
        <br />
        <asp:Label ID="Label2" runat="server" Text=""></asp:Label>
        <br />
        <asp:Button ID="Button1" runat="server" Text="Previous" />  
        <asp:Button ID="Button2" runat="server" Text="Next" /> 
        <asp:Button ID="Button3" runat="server" Text="Play" />
        </asp:Panel>
    </div>
    </form>
</body>
</html>后台代码using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;namespace LinqTest
{
    public partial class SlideShowExtenderTest : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {        }
        [System.Web.Services.WebMethodAttribute(),
            System.Web.Script.Services.ScriptMethodAttribute()]
        public static AjaxControlToolkit.Slide[] GetSlides(string key)
        {
            return new AjaxControlToolkit.Slide[]{
                new  AjaxControlToolkit.Slide("images/cover.jpg","The One","This is p picture of one"),
                new  AjaxControlToolkit.Slide("images/Front.jpg","The Second","This is p picture of Second"),
                new  AjaxControlToolkit.Slide("Ratt - Reach For The Sky-back.jpg","The Third","This is p picture of Third")
            };
        }
    }
}
效果没有出来，点击页面的按钮，就会出js错误行：326
Char：13
错误：'this._slides.length'为空或不是对象
代码：0
请教高手解决，谢谢！