TDN Platformer-like movement problem

This isn't actually a problem that I'm having with the TDN Platformer tutorial, just with bits of code I've nicked out of it for my own use. I'm using T2D v1.1 and here are the relevant bits of code:

function initPlayerControls()
{
	new ActionMap(playerMoveMap);
	playerMoveMap.bind(keyboard, left, "playerLeft");
	playerMoveMap.bind(keyboard, right, "playerRight");
	playerMoveMap.push();
}

function playerLeft(%val)
{
	$moveLeft = %val;
}

function playerRight(%val)
{
	$moveRight = %val;
}

 function updatePlayer()
{
   %moveX = ($moveRight - $moveLeft) * $player.speed;
   $player.setLinearVelocityX(%moveX);
}

Judging from the descriptive text in TDN, this method of keyboard control should help get over pressing two opposing directions at once and then coming to a halt once you release one of the keys.

However, from what I can make out, all it does is stop the player dead in his tracks as soon as he presses in the opposite direction (whilst still holding down the previous direction key).

The maths simply cancel each other out if you're moving you're moving one way and try to move directly opposite.

Have I missed a step?