Can't get AS3 KeyboardEvent to work

When I use the code below to capture a KeyBoard event I get the following error:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at Racer$iinit()
at MyMain$iinit()

Can anyone tell me why?

-----Code------

package {
import flash.ui.Keyboard;
import flash.display.*;
import flash.events.*;
import RaceCar;



public class Racer extends Sprite {
//
public function Racer():void {

stage.addEventListener(KeyboardEvent.KEY_DOWN, keyDownListener);

}
private function keyDownListener(e:KeyboardEvent):void {
trace("keyDownHandler");

}
}

}