window.open为什么在IE7中不好用呢，提示新窗口为空或不是对象，但在IE6中就没问题

<%@ page contentType="text/html; charset=gb2312" %>
<html>
<head>
<title>test</title>
</head>
<body>
<script>
var newWin = window.open("/MDSS/portlets/login.jsp",'newwindow',"status=yes,toolbar=yes,menubar=yes,location=yes,channelmode=0,directories=0,resizable=yes,titlebar=yes,top=0, left=0",null,false);
newWin.moveTo(0,0);
newWin.resizeTo(window.screen.availWidth,window.screen.availHeight);
window.opener=null;
window.close();
</script></body>