帮帮忙，看看这是什么错误

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<HEAD>
<title>Map</title>
<meta name="GENERATOR" content="Microsoft Visual Studio .NET 7.1">
<meta name="CODE_LANGUAGE" content="Visual Basic .NET 7.1">
<meta name="vs_defaultClientScript" content="JavaScript">
<meta name="vs_targetSchema" content="http://schemas.microsoft.com/intellisense/ie5">
<script language="JScript.Encode">
var st = false
var x1
var x2
var y1
var y2
var stx
var sty
function MouseDown(){
st = !st
if (st){
div1.style.posLeft=-5000
div1.style.width= 0
div1.style.height = 0
stx = event.clientX-2
sty = event.clientY-2
window.parent.document.getElementById("zb").value =''
}
}
function Move(){
var xx1
var yy1
if (st){

if (event.clientX >= stx){
x1 = stx
x2 = event.clientX - stx
}
else{
x1 = event.clientX
x2 = Math.abs(event.clientX - stx)
}

if (event.clientY >= sty){
y1 = sty
y2 = event.clientY - sty
}
else{
y1 = event.clientY
y2 = Math.abs(event.clientY - sty)
}

if (x1 + x2 > document.getElementById("main").offsetWidth - 40)
x2 = document.getElementById("main").offsetWidth - x1 - 40

try{
div1.style.posLeft=x1 + document.body.scrollLeft
div1.style.posTop=y1 + document.body.scrollTop
div1.style.width=x2
div1.style.height=y2
xx1=x1 + document.body.scrollLeft
yy1=y1 + document.body.scrollTop
//alert( xx1 + ',' + y1 + ',' + x2 + ',' + y2 )
window.status='选定地图区域: '+ xx1 + ',' + yy1 + ' ' + x2 + "x" + y2
window.parent.document.getElementById("zb").value = xx1*14.92 + ',' + yy1 * 14.9183 +

',' + (xx1+x2) * 14.92 + ',' + (yy1+y2) * 14.9183
}
catch(e)
{}
}
}
function Loading(){
document.body.scrollTop =590
document.body.scrollLeft =280
window.status='等待选定地图区域'
}
</script>
</HEAD>
<body oncontextmenu="self.event.returnValue=false" MS_POSITIONING="GridLayout" onmousemove="Move()"
onload="Loading()" style="CURSOR: url('/images/mouse.ani')">
<form id="Form1" method="post" runat="server">
<img src="file:///C|/Documents%20and%20Settings/Administrator/My%20Documents/My%20Pictures/aa.jpg" width="283" height="212" border="1" id="main"

style="BACKGROUND:#cccccc;LEFT:0px;POSITION:absolute;TOP:0px"
onmousedown="javascript:MouseDown();">
<div onmousedown="javascript:MouseDown();" id="div1" style="BORDER-RIGHT: green 1px solid;

BORDER-TOP: green 1px solid; FONT-SIZE: 1pt; BACKGROUND: teal; FILTER: alpha(opacity=40); LEFT: -500px; BORDER-LEFT: green

1px solid; WIDTH: 60px; BORDER-BOTTOM: green 1px solid; POSITION: absolute; TOP: 18px; HEIGHT: 37px"></div>
</form>
</body>
</HTML>
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报错：window.parent.document.getElementById("...")为空或不是对象