[jQuery] AJAX in IE 6 providing "Object does not support this property or method"

[Synoiz]

Here is the function I have created to handle AJAX requests on top of
the latest JQuery release:

// Portal for an AJAX query
function ajax_query(url, data) {

        var ajax_return = $.ajax({
                url: '/scripts/ajax/' + url + '?' + data,
                async: false
        }).responseText;

        return ajax_return;

}

I call it with something like:

alert(ajax_query("script.php","variable1=true&variable2=false"));

Now this code simply does not work in IE 6, providing a "Object does
not support this property or method" error.
I've worked it down to either this function or JQuery itself that is
causing the error as if I delete the calls to this custom function
then the script does not produce any errors.

Any help would be appreciated, thank you :D