asp.net2.0脚本的问题

//后台
using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;public partial class Default3 : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        txtVolume_project.Attributes.Add("onblur", "sum(Text1,Text2,Text3)");
        txtCustomer_share.Attributes.Add("onblur", "sum(Text1,Text2,Text3)");
    }
}//前台<%@ Page Language="C#" MasterPageFile="~/MasterPage.master" AutoEventWireup="true" CodeFile="Default3.aspx.cs" Inherits="Default3" Title="Untitled Page" %>
<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
         <script type="text/javascript">
    function sum()
{
    if(this.document.all('txtVolume_project').value!="" && this.document.all('txtCustomer_share').value!="")
    {
       var n1=eval(this.document.all('txtVolume_project').value);
      var n2=eval(this.document.all('txtCustomer_share').value);
      var n3=n1+n2;
       this.document.all('lblCustomer_volume').value=n3;
    }
}
    </script>
    <table style="width: 688px; height: 158px">
        <tr>
            <td style="width: 100px">
            </td>
            <td style="width: 100px">
            </td>
            <td style="width: 100px">
            </td>
        </tr>
        <tr>
            <td style="width: 100px">
                <asp:TextBox ID="txtVolume_project" runat="server"></asp:TextBox>
            </td>
            <td style="width: 100px">
                &nbsp;<asp:TextBox ID="txtCustomer_share" runat="server"></asp:TextBox></td>
            <td style="width: 100px">
                <asp:TextBox ID="lblCustomer_volume" runat="server"></asp:TextBox></td>
        </tr>
        <tr>
            <td style="width: 100px">
                <input id="Text1" runat="server" type="text"  onblur="sum(Text1,Text2,Text3)"/></td>
            <td style="width: 100px">
                <input id="Text2" runat="server" type="text" onblur="sum(Text1,Text2,Text3)"/>
            </td>
            <td style="width: 100px">
                <input id="Text3" runat="server" type="text" /></td>
        </tr>
    </table>
   
</asp:Content>//模板前台<%@ Master Language="C#" AutoEventWireup="true" CodeFile="MasterPage.master.cs" Inherits="MasterPage" %><!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>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        &nbsp;<asp:contentplaceholder id="ContentPlaceHolder1" runat="server">
        </asp:contentplaceholder>
    </div>
    </form>
</body>
</html>运行就说控件ID不能识别,求助!!!!!