document event not firing if used in a non-frames environment?

[SteveSu]

Hi!
I have a script which looks like this. If the script is added to a page
which is in a frameset it works fine, but if I use it in a separate
page "outside the frameset" the event is not fired at all. Why is that?

<script language=javascript>

document.onkeydown=function GoBack() {
if(event.keyCode == 27) {
__doPostBack('', 'GoBack');
}
}

</script>

Thanks in advance
/ Steve