IE9 jqXHR.responseXML returns "undefined"

IE9 jqXHR.responseXML returns "undefined" while FireFox 3.6.13 correctly returns: [object XMLDocument]

JavaScript

$.ajax({
    url: "/echo/xml/",
    dataType: "xml",
    type: "post",
    data: {
        xml: "<rooty><test>123</test></rooty>"
    },
    complete: function(jqXHR, a, b) {
        alert("jqXHR.responseText = " + jqXHR.responseText);
        var xml = jqXHR.responseXML;
        alert(xml.getElementsByTagName("test")[0].childNodes[0].nodeValue);
     }
});