Tween Alpha Values

I want to change the value of my material's alpha channel over time. I can't seem to get it to work.
I have attached my code. It's throwing an error on line 34 saying that TWEEN is undefined: var tween = new TWEEN.Tween(tweenStartAlpha)
Any help would be greatly appreciated!

I thought tweening might be the best route, but I'm open to other suggestions on how to change a value over time.

// -----JS CODE-----

// @input Asset.Material tux

// @input vec4 startAlpha = (255, 255, 255, 255)

// @input vec4 goalAlpha = 255, 255, 255, 0)

// @input float time

var tweenStartAlpha = {

r: script.tux.getPass(0).baseColor.r,

g: script.tux.getPass(0).baseColor.g,

b: script.tux.getPass(0).baseColor.b,

a: script.tux.getPass(0).baseColor.startAlpha

};

var tweenGoalAlpha = {

r: script.tux.getPass(0).baseColor.r,

g: script.tux.getPass(0).baseColor.g,

b: script.tux.getPass(0).baseColor.b,

a: script.tux.getPass(0).baseColor.goalAlpha

};

var tween = new TWEEN.Tween(tweenStartAlpha)

.to(tweenGoalAlpha, script.time * 1000.0)

.onUpdate(function() {

updateAlpha(tweenStartAlpha);

})

.start();

// Here's where the values returned by the tween are used

// to drive the alpha of the SceneObject

function updateAlpha(alpha) {

//var material = script.tux.getMaterial

//material.getPass(0).baseColor(new vec4(alpha.r, alpha.g, alpha.b, alpha.a);

script.tux.getPass(0).baseColor = new vec4(alpha.r, alpha.g, alpha.b, alpha.a);

}

// On update, update the Tween engine

function onUpdateEvent() {

TWEEN.update();

}

// Bind an update event

var updateEvent = script.createEvent("UpdateEvent");

updateEvent.bind(onUpdateEvent);