[jQuery] parseerror on GET from window.location

[jacktanner]

I'm trying to do an AJAX GET.

            var q_url = window.location.toString();
            $.ajax({
                type: 'GET',
                        url: q_url,
                        cache: false,
                        dataType: 'json',
                        success: function(response, textStatus) { ... },
                        error: function (xhr, textStatus, errorThrown) {
                          alert(textStatus);
                        }
            });

This triggers the error callback with textStatus == "parseerror". If I
hardcode q_url, everything works. Stepping through with Firebug shows
that q_url has the same value no matter if it's hardcoded or set via
window.location.toString() (or window.location.href or
window.location.pathname, which are all supposed to be strings
according to https://developer.mozilla.org/En/DOM/Window.location ).
The browser is Firefox 3.0.11. Any ideas?

A separate issue is that no matter whether the GET succeeds or fails,
instead of going to q_url, it goes to a url like q_url + '?
_=1245297612818'  (according to Firebug 1.3). What gives?