error in ie 'documentElement' is null or not an object

this library iam using

/**
* Returns an XMLHttp instance to use for asynchronous
* downloading. This method will never throw an exception, but will
* return NULL if the browser does not support XmlHttp for any reason.
* @return {XMLHttpRequest|Null}
*/
function createXmlHttpRequest() {
try {
if (typeof ActiveXObject != 'undefined') {
return new ActiveXObject('Microsoft.XMLHTTP');
} else if (window["XMLHttpRequest"]) {
return new XMLHttpRequest();
}
} catch (e) {
changeStatus(e);
}
return null;
};

/**
* This functions wraps XMLHttpRequest open/send function.
* It lets you specify a URL and will call the callback if
* it gets a status code of 200.
* @param {String} url The URL to retrieve
* @param {Function} callback The function to call once retrieved.
*/
function downloadUrl(url, callback) {
var status = -1;
var request = createXmlHttpRequest();
if (!request) {
return false;
}

request.onreadystatechange = function() {
if (request.readyState == 4) {
try {
status = request.status;
} catch (e) {
// Usually indicates request timed out in FF.
}
if ((status == 200) || (status == 0)) {
callback(request.responseText, request.status);
request.onreadystatechange = function() {};
}
}
}
request.open('GET', url, true);
try {
request.send(null);
} catch (e) {
changeStatus(e);
}
};

/**
* Parses the given XML string and returns the parsed document in a
* DOM data structure. This function will return an empty DOM node if
* XML parsing is not supported in this browser.
* @param {string} str XML string.
* @return {Element|Document} DOM.
*/
function xmlParse(str) {
if (typeof ActiveXObject != 'undefined' && typeof GetObject != 'undefined') {
var doc = new ActiveXObject('Microsoft.XMLDOM');
doc.loadXML(str);
return doc;
}

if (typeof DOMParser != 'undefined') {
return (new DOMParser()).parseFromString(str, 'text/xml');
}

return createElement('div', null);
}

/**
* Appends a JavaScript file to the page.
* @param {string} url
*/
function downloadScript(url) {
var script = document.createElement('script');
script.src = url;
document.body.appendChild(script);
}

yeah sure, actually iam getting data in the form of xml from ajax file to show it on the google map as markers.it is working fine in chrome and firefox but not in ie ,somtimes it works and there is another error comes all the time it says
'dx' is null or not an object