为什么输出结果不对？
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>兄弟节点</title>
</head>
<body>
<ul id="she">
<li id="a">nishi</li>
<li id="b">wodetian</li>
<li>hahahh</li>
</ul>
<ul id="her">
<li id="c">jiusuan</li>
<li id="d">shibushi</li>
<li>gegge</li>
</ul>
<script type="text/javascript">
var x=document.getElementsByTagName("li")[0];
var y=x.nextSibling;
document.write(x.innerHTML+"<br />");
document.write(y.innerHTML);
</script>
</body>
为什么y.innerHTML输出的结果是undefind
