Prototype Event.observe Onclick Not Returning NodeName In IE

I have the following script, which uses prototype.js and is called when the page loads:

function getAllDefs() {
allDefs = document.getElementsByClassName('def');
var defs = $A(allDefs);
defs.each(function(def){
Event.observe(def, "click", listElement, false);
});
}
It adds an onclick event handler to each element with a 'def' class. Clicking the element then calls the "listElement" function, defined as follows:

function listElement(e) {
alert(this.nodeName);
}

This works great in Firefox, returning "SPAN" for the element. However, in IE6, "undefined" is returned. I've tried nodeType and innerHTML as well, and all are "undefined" in IE6. Anyone know what the deal is here with IE? It's somewhat hard to tell since IE's responses are so vague.