hitTest errors between attached movieclip

programming flash
1

i’ve checked this serveral times, there is nothing wrong with my actionscript, but still hitTest returns undefined between movieclips put onto the stage using attachMovie();

here’s my code:

thingOne = function() {};
thingTwo = function() {
	this._x = 100;
	this._y = 100;
};
//
Object.registerClass("one", thingOne);
Object.registerClass("two", thingTwo);
//
thingOne.prototype.onEnterFrame = function() {
	//make sure that both movieclips are valid movieclips
	trace(this);
	trace(_root.two);
	//
	trace(this.hitTest(_root.two));
}
//
_root.attachMovie("one", "one", 1);
_root.attachMovie("two", "two", 2);

is this is a bug? was it left out on purpose? am i wrong?
why is this?