XMLHttpRequest.reponseXML works on one page, but not another

Question

In creating a google chrome extension, my XMLHttpRequest.responseXML works fine on my background.html page, but not my popup.html page.

Here is the code from the background.html page:

var request = new XMLHttpRequest();
request.open("GET","comics.xml",false);
request.send();
domXML = request.responseXML;
var xmlBookmarks = domXML.getElementsByTagName("bookmark");

and on the popup.html page:

var request = new XMLHttpRequest();
request.open("GET","comics.xml",false);
request.overrideMimeType('text/xml');
request.send();
return request.responseXML;

then the popup.html page returns a null object. If I use responseText, then I get the string as I should get it, but the xml file is perfectly formed.

Thanks in advance!