Flex :: Make A Variation On Array For A Very Specific Purpose?

I'm trying to make a variation on Array for a very specific purpose. When I have the following:

public class TileArray extends Array {
// Intentionally empty - I get the error regardless
}

Why can't I do this?

var tl:TileArray = [1,2,3];

despite the fact that I can do this

var ar:Array = [1,2,3];

The error I receive is this: Implicit coercion of a value with static type Array to a possibly unrelated type