Trying to use addClass removeClass

I try and run the demo on this page within IE...
http://docs.jquery.com/UI/Effects/addClass
I am getting "Invalid Property Value" on this line:
jQuery.extend( jQuery.fx, {
...
step:
...
fx.elem.style[ fx.prop ] = fx.now + fx.unit; // fx.now ==
NaN
Basically, the debugger NaN. Am I the only person having this
problem?
Thanks,
Chris