Anyone else have trouble with onmouseover in IE9? Not inline, but in a external JS doc. I know, jQuery can do it, but works in IE7-8, FF, Chrome

I'm trying to do a simple rollover. It works in IE7, IE8, FF, Chrome, but IE9 is a no-go.

var FB = document.getElementById('facebook_logo');

FB.onmouseover=function(){FB.src='images/facebook.png';};

<img src="images/facebook_mono.png" width="20" height="20" alt="Facebook" border="0" id="facebook_logo">

My code validates, and the JS looks good in JSLint. WTF?

Even stackoverflow is ignoring me on this one.