Why does this jQuery Ajax call fail ONLY in IE9 (Even works fine in IE8 and IE7)

I have a website where I make an ajax call like this:

            // perform an ajax request to generate a new subscriber account
            jQuery.ajax({
                type: 'POST',
                url: '/index.php?option=com_content&view=article&id=45&tmpl=component',
                data: postVars,
                success: handleResponse,
                error: function(jqXHR, textStatus, errorThrown) {
                    alert ('response: ' + jqXHR.responseText);
                    alert ('code: ' + jqXHR.getResponseHeader('X-Subscriber-Status'));
                }
            });

The page being requested doesn't do anything but return a single digit status code and it works fine with every browser except for IE9. The script is hosted on the same domain as the page it's requesting so there shouldn't be any issue with cross domain scripting.

I finally tracked down what's happening but I don't know how to fix it. It turns out that the Ajax request completes successfully with no problems. I verified this by using Fiddler to look a the network traffic. The server responds to the request with an HTTP status code of 200 and the body doesn't contain anything but a single digit response code. At this point jQuery executes the error handler, NOT the success handler as would be expected (in IE9). Why does it do this and how can I prevent it?? This ONLY happens in IE9. Even IE8 and IE7 work just fine with the exact same code! As you can see I finally resorted to detecting IE9 and using an XmlHttpRequest object to do it (which works just fine by the way).

This seems like a jQuery bug to me but I can't find any mention of it in my searches. Am I really the only one experiencing this odd behavior?