Garbage Collection with MovieClips

var a:MovieClip = new MovieClip();
var dict:Dictionary = new Dictionary(true);
dict[a] = true;
a = null;

How come the movieclip never gets cleared, but if i change MovieClip to Object it gets cleared, im using this code to check if its cleared or not

function sup():void {
	new Wait(30, sup);
	for(var key:* in dict) {
		trace("data is still there");
	trace("data was cleared");