I am trying to make accelerated movement, but I am running into a problem that, for the life of me, I cannot understand.

My class definition:

public class Player extends MovieClip
{
    private var stageRef:Stage;
    private var key:KeyObject;
    private var acceleration:int = .5;
    private var curSpeed:int = 0;

    public function Player(stageRef:Stage)
    {
        this.stageRef = stageRef;

        addEventListener(Event.ENTER_FRAME, enterFrame);
        key = new KeyObject(stageRef);
    }

    public function enterFrame(e:Event) : void
    {
        if(key.isDown(key.RIGHT)) {
            x += 5;
        }
    }
}

This works to move my position in the x direction at a constant rate.

However, if I change enterFrame to

    public function enterFrame(e:Event) : void
    {
        if(key.isDown(key.RIGHT)) {
            x += acceleration;
        }
    }

No movement occurs.

Is there something going on in the event I do not understand? Why is it that I can have x increased by a constant value but not a constant value as defined in a variable in the class? Is it a scope issue?