执行时总是提示“JavaScript 运行时错误: 无法获取未定义或 null 引用的属性“UI”,请各位帮忙，谢谢！

出错位置：
Sys.Application.add_init(function() {
    $create(Sys.Extended.UI.AutoCompleteBehavior, {"delimiterCharacters":"","firstRowSelected":true,"id":"MainContent_AutoCompleteExtender1","minimumPrefixLength":1,"serviceMethod":"GetData","servicePath":"autoCompExtenderServer.asmx"}, null, null, $get("MainContent_txtBox1"))

网页：
<%@ Page Title="主页" Language="C#" MasterPageFile="~/Site.master" AutoEventWireup="true"
    CodeBehind="Default.aspx.cs" Inherits="WebApplication3._Default" %>

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %>

<asp:Content ID="HeaderContent" runat="server" ContentPlaceHolderID="HeadContent">
    <style type="text/css">
        #Text1
        {
            margin-bottom: 0px;
        }
    </style>
</asp:Content>
<asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent">

    <p>
        <asp:TextBox ID="txtBox1" runat="server"></asp:TextBox>
        <asp:ScriptManager ID="ScriptManager1" runat="server">
        </asp:ScriptManager>
    </p>
    <asp:AutoCompleteExtender ID="AutoCompleteExtender1" runat="server"
     TargetControlID="txtBox1"                                         
     ServicePath="autoCompExtenderServer.asmx"
     ServiceMethod="GetData"
     CompletionSetCount="10"
     FirstRowSelected="true"
     MinimumPrefixLength="1">
    </asp:AutoCompleteExtender>

</asp:Content>

WebService:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Services;
using System.Runtime.Serialization;
using System.Web.Script.Serialization;
namespace WebApplication3
{
    /// <summary>
    /// autoCompExtenderServer 的摘要说明
    /// </summary>
    [WebService(Namespace = "http://tempuri.org/")]
    [WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
    [System.ComponentModel.ToolboxItem(false)]
    // 若要允许使用 ASP.NET AJAX 从脚本中调用此 Web 服务，请取消对下行的注释。
    [System.Web.Script.Services.ScriptService]
    public class autoCompExtenderServer : System.Web.Services.WebService
    {

        [WebMethod]
        public string[] GetData(string prefixText, int count)
        {
            string[] data = new string[10000];
            for (int i = 0; i < data.Length; i++)
                data[i] = i.ToString("0000");
            return data.Where(p => p.IndexOf(prefixText) >= 0).Take(count).ToArray();
        }
    }
}