打开新窗口

我按照文档写了几行代码，就是打开新窗口，<script src="js/mui.min.js"></script>
<link href="css/mui.min.css" rel="stylesheet"/>
<script type="text/javascript" charset="UTF-8">
mui.init();
mui.plusReady(function(){
document.getElementById('openwin').addEventListener('tap', function() {
//打开关于页面
mui.openWindow({
url: 'new_window.html',
id:'new_window'
});
});
});
</script>
报错：TypeError: Result of expression 'window.plus' [undefined] is not an object. at undefined:1

这是什么情况?