zIndex doesn't animate

app.js

var win = Ti.UI.createWindow({
	backgroundColor: '#fff',
	fullscreen: false,
	exitOnClose: true
});

var view1 = Ti.UI.createView({
	height: 200,
	width: 200,
	left: 30,
	top: 30,
	backgroundColor: '#00a',
	opacity: 0.5,
	zIndex: 1
});
var view2 = Ti.UI.createView({
	height: 200,
	width: 200,
	left: 10,
	top: 10,
	backgroundColor: '#a00',
	opacity: 0.25,
	zIndex: 2
});
var view3 = Ti.UI.createView({
	height: 200,
	width: 200,
	left: 10,
	top: 10,
	backgroundColor: '#a00',
	opacity: 0.25,
	zIndex: 3
});
var view4 = Ti.UI.createView({
	height: 200,
	width: 200,
	left: 10,
	top: 10,
	backgroundColor: '#a00',
	opacity: 0.25,
	zIndex: 4
});

win.add(view1);
win.add(view2);
win.add(view3);
win.add(view4);

win.open();

win.addEventListener('click', function(e) {
	var anim = Ti.UI.createAnimation({
		duration: 4000,
		zIndex: 5
	});
	anim.addEventListener('complete', function(e) {
		alert('done');
	});
	view1.animate(anim);
});