I got a ajax error "No Transport" in IE6, not a cross site issue

I got a ajax error "No Transport" in IE6, not a cross site issue

I got a ajax error, 'No Transport', and I just POST some data to one part of my site, the action field in form is :'/shopping-cart/item'

$.ajax({
                type:'POST',
                url:'/shopping-cart/item',
                data: widget.serialize(),
                dataType:'html',
                success: function(html) {location.href=html},
                error: function(xhr, msg, thrown) {alert(thrown)},
                statusCode:{
                    403:function() {alert(‘xxx’');}
                }
            });