Window.open Not Working From Inside User Defined Function

I'm trying to use window.open from inside a user defined function, and
it's not working.

Mountain Man
============

<script>

function openFootnote(contentURL) {
window.open('contentURL','footnotes','height=180,w idth=160');
}

</script>

I've tried calling on the function 2 different ways, as shown below.
Nothing happens either way.

<a href="javascript:(void)" onClick="openFootnote(fn1.html);">...</a>

<a href="javascript:(void)" onClick="return
openFootnote(fn1.html);">...</a>