jQuery UI Show Method fails in IE

Question

I'm using jQuery 1.4.2 and jQuery UI 1.8.4, on the latest versions of IE and Firefox, under Windows XP. Firefox is fine with the following code:

var $container = jQuery('<div></div>');
jQuery('body').append($container);
$container.show('size', {to: {width: 640} }, 'slow');
$container.show('size', {to: {height: 500} }, 'slow', function() {...} );

However, IE8 fails. On line 11046 of jQuery-ui.js, the value of el.from.fontSize is "NaNpx", which causes the call to css() to throw an exception with the message: "Invalid argument."

By combining the two calls to show() (which I don't want to do), and giving the div a non-zero width and height, I can get past the errors, but it's ruining the effect I'm after. Also, why is Firefox (and also Chrome, Safari and Opera) happy with the code, but not IE?