Yes, I know you shouldn't animate undefined props! But a minor change can prevent IE from borking when this happpens.
Sample file which shows the error:
<html>
<head>
<script type="text/javascript" src="../rel/jquery-1.1.3.1.js"></script>
<script type="text/javascript">
$(function() {
$('body').animate({top: 50}, 'fast');
});
</script>
</head>
<body><div></div></body>
</html>
This change to fx.js will fix the problem:
Index: fx.js
===================================================================
--- fx.js (revision 2783)
+++ fx.js (working copy)
@@ -336,7 +336,7 @@
jQuery.each( prop, function(name, val){
var e = new jQuery.fx( self, opt, name );
if ( val.constructor == Number )
- e.custom( e.cur(), val );
+ e.custom( e.cur() || 0, val );
else
e[ val == "toggle" ? hidden ? "show" : "hide" : val ]( prop );
});