我从。cs文件中返回一个类似于xml的字符串，为什么转化不成xml文档啊！！求助

var root=xmlhttp.responseText;
                     
                     alert(root);
                     var doc= new ActiveXObject("Microsoft.XMLDOM");
                     doc.loadXML(root);
                     alert(doc);
                     var rootNode = doc.getElementsByTagName("root");
                     alert(rootNode);
                     if(rootNode!=null)
                     {
                        var buff = doc.getElementsByTagName("Cell_Content").InnerText;
                        alert(buff);
                      ｝
buff为空啊