Errore Ajax: help

[sally3000]

ho questo pezzo di funzione che mi fa errore nella segnata con la freccia ----->, perchè?

this.xmlhttp.onreadystatechange = function() {
switch (self.xmlhttp.readyState) {
case 1:
self.onLoading();
break;
case 2:
self.onLoaded();
break;
case 3:
self.onInteractive();
break;
case 4:
self.response = self.xmlhttp.responseText;
self.responseXML = self.xmlhttp.responseXML;
-----> self.responseStatus[0] = self.xmlhttp.status;
self.responseStatus[1] = self.xmlhttp.statusText;

if (self.execute) {
self.runResponse();
}

if (self.elementObj) {
elemNodeName = self.elementObj.nodeName;
elemNodeName.toLowerCase();
if (elemNodeName == "input"
|| elemNodeName == "select"
|| elemNodeName == "option"
|| elemNodeName == "textarea") {
self.elementObj.value = self.response;
} else {
self.elementObj.innerHTML = self.response;
}
}
if (self.responseStatus[0] == "200") {
self.onCompletion();
} else {
self.onError();
}

self.URLString = "";
break;
}
};