Prototype 怎么取得xml对像啊?

[accpdingyi]

function searchGrid(page,pars)
{
var url =page;
var pars = pars;
var myAjax = new Ajax.Request(url,{method: 'get', parameters: pars,onComplete:UpdageGrid});}
function UpdageGrid(originalRequest)
{var xmlObj=originalRequest.responseXml;
var nodeList=xmlObj.getElementsByTagName("aa");
-------------------------------------------------
alert(nodeList.length);

}
为什么nodeList.length总是等于0啊,已经查过了xml文档,有aa结点.是我的xmlobj创建有问题吗?
请帮忙

[rob123]

alert(xmlObj.xml);看看。