Initialising an array of mcs variable problem

[sfpiano]

var str:String;
for(i=0; i<13; i++) {
	for(j=0; j<21; j++) {
		theScene["mask_"+i+"_"+j].onRollOver = function() {
			str = "mask_"+i+"_"+j;
			trace(str);
			theScene[str].gotoAndPlay(2);
		}	
	}
}

Everytime I run this it prints out mask_13_21, no matter which mask I'm actually on.