ActionScript 3.0 :: Wasd Not Working?

I'm trying to let my ship move with WASD.I've got it to work with the arrow keys, no problem.It seems to be the problem that my event isn't triggered when i press W, A, S or D. Because,when i place a trace(event.keyCode), it doesn't trace anything.I'm using this within a class.Here's my (part of the) code:

Code:
public function ship() {
stage.addEventListener(KeyboardEvent.KEY_DOWN, keysDown);
stage.addEventListener(KeyboardEvent.KEY_UP, keysUp);[code]..

In the code above, the W key doesn't work. But the left, right and down arrow keys work fine.If i change those to the corresponding key values of a, s and d.It doesn't work anymore.