__EVENTTARGET' 为空或不是对象

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="WebForm1.aspx.cs" Inherits="WebApplication1.WebForm1" %><!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>无标题页</title>
    <script type="text/javascript">
    
      function pageLoad() {
      }
    
    </script>
     <base target="_self"> 
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <asp:ScriptManager ID="ScriptManager1" runat="server" />
        <asp:UpdatePanel ID="UpdatePanel1" runat="server">
            <ContentTemplate>
                <asp:Timer ID="Timer1" runat="server" Interval="1000">
                </asp:Timer>
                <br />
                <br />
            </ContentTemplate>
        </asp:UpdatePanel>
    </div>
    <p>
        <asp:Label ID="Label1" runat="server" Text="Label"></asp:Label>
    </p>
    </form>
</body>
</html>===================================using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;namespace WebApplication1
{
    public partial class WebForm1 : System.Web.UI.Page
    {
                protected void Page_Load(object sender, EventArgs e)
        {        }        protected void Timer1_Tick(object sender, EventArgs e)
        {            Label1.Text = Label1.Text = "Panel refreshed at: " + DateTime.Now.ToLongTimeString();
        }
    }
}===============================
为何总是显示 __EVENTTARGET' 为空或不是对象 ？然后只能中断？