dom取xml节点值问题

Question

        var xmlDoc = new ActiveXObject("Microsoft.XMLDOM");
//         xmlDoc.loadXML(xmlHttp.responseText);
                xmlDoc = xmlHttp.responseXML;
        var i = xmlDoc.getElementsByTagName("set")[0].firstChild.nodeValue;
        alert(i);
    }firstChild取出是object,但是firstChild.nodeValue取出是null,请教怎么回事?<?xml version="1.0" encoding="gb2312"?>
<set>
  <aa>Hello</aa>
  <bb>123456</bb>
</set>