Connot Set Handlers On XMLHttpRequest Instance In FF1.5

I have the following code running in FireFox 1.5 (a remote XUL
application):

var req = new XMLHttpRequest();
// execution will block until request is
completed
// third param set to false makes request
synchronous
req.open(strMethod, uriTarget, false);
if(strMethod == "POST") {

req.setRequestHeader('Content-Type','application/x-www-form-urlencoded;
charset=UTF-8');
}
req.onreadystatechange = function () {
if (req.readyState == 4) {
if(req.status == 200) {
strResult = req.responseText;
xmlResult = req.responseXML;
}
else {
strLastError = uriTarget + " returned http error code " +
req.status + "
" + req.statusText;
if(intVerbosity > 1) throw new Error(strLastError);
}
}
}
req.onerror = function() {
strLastError = uriTarget + " fetch failed while " +
arrReadyState[req.readyState];
if(intVerbosity > 1) throw new Error(strLastError);
}
req.send(strRequest);

By the time it gets to req.send() (at the bottom) all the callback
functions like req.onerror still have values of null. I've verified
this using the Venkman debugger with req as a watch variable.

the code is clearly assigning functions to those callback function
objects. Any idea why they are all turning up null?