Event Handling

I am having some trouble tracking key code events.  I can get it to work in
IE but not in Fire Fox.

<INPUT TYPE="text" NAME="LookUpKey" onkeyup = FindMatch()>

This calls the function "FindMatch()"

function FindMatch(evt) {

//this next line should detect either the IE window object property or the
W3C DOM event object as a parameter.  However it comes up null in FireFox.

   evt = (evt) ? evt : ((window.event) ? event : null);

//as a result this next line crashes because evt = null and has no
properties.

   var charCode = (evt.charCode) ? evt.charCode : ((evt.keyCode) ?
evt.keyCode : ((evt.which) ? evt.which : 0));
   if (evt) {
      if ((charCode > 64 && charCode < 91) || charCode == 8 || charCode ==
32) {

As you can see from the code I want to trap alpha char, space and backspace
keys only.  Any suggestions?