Prototype function works in flash, not online

Like the title says, I found a cool prototype function here in the forums that plays a movieclip backwards:

MovieClip.prototype.backwards = function() {
	this.onEnterFrame = function() {
		if (this._currentframe != 1) {
		} else {
			delete this.onEnterFrame;

I have the protoype defined on the main timeline of my main file. I call the function on press of a button from an external movieclip after it’s loaded into main.

When testing it in flash it works beautifully. When I test it online it doesn’t work at all. Other code on the button being called at the same time does work (like unloading a movieclip on the main timeline).

Any thoughts on why?