[Moo] fx.tween and IE8

[mmjaeger]

hello
the following code works fine in Safari and Firefox - however, it
doesn't work in IE 8 - I'm I doing anything wrong - is there a
workaround?
Thank you in advance for your input.

var fxTween = new Fx.Tween(container, {
                        property: 'opacity',
                        duration: 2500,
                        complete: $empty
                }).set(0);

                // load background
                var images = new Asset.images([url], {
                        onComplete: function() {
                                fxTween.start(0, 1);
                        }.bind(this)
                })