'Lost' Function Return Value: What Am I Doing Wrong?

I am relatively new to js, but I did think I was starting to get the
hang of it. Then this happened...

I have a form with an onsubmit event handler:
<form id="uploadForm" method="post" action="..."
onSubmit="checkDates()">

The event handler does some minor validation, then returns true or
false:

function checkDates(y, m, d) {

if (endDate.getTime() >= startDate.getTime())
return true;

alert("Start date must precede end date");
return false;
}

(The arguments to the function are used when it is called elsewhere,
not as onsubmit.)

I also have a library class which needs to process the form submit, so
it hooks onsubmit like this:
MyClass.setOnSubmit = function(listId) {
var list = document.getElementById(listId);
var form = list.form;

var f = form.onsubmit;
if (typeof f == "function") {
form.oldOnSubmit = f;
form.onsubmit = function(){
var ok = this.oldOnSubmit();
if (ok)
return MyClass.onsubmit(listId);
else
return false;
};
}
else
form.onsubmit = function(){MyClass.onsubmit(listId);};
}

My problem is that the value returned from oldOnSubmit and stored in ok
appears as 'void'. This happens in IE 6 and in FireFox 1.07. Can anyone
explain what's happening?