[jQuery] ajax error handling troubles

[Davide]

                $.ajax({
                        async: true,
                        url: testingURL,
                        success: function () {
                                location.reload();
                        },
                        error: function (XMLHttpRequest, textStatus, 
errorThrown) {
                                console.log('error throw');
                                console.log(errorThrown);
                        }
                });

the requested page can return  404 or 403 error codes
but the errorThrown variable is ALWAYS undef...
any idea ??