iTween call producing NullReferenceException

Hey, guys.

When I run this short script, I'm getting a NullReferenceException error. I have no idea why. The error points to this line in iTween:

 if(tweenArguments.Contains("looktarget") && isRunning){

I know it's the "MoveBy" call, because the other part is working correctly--but no luck with the first bit.

Here's the script. Any ideas about what I'm doing wrong?

UPDATE: Tried using even a basic MoveBy iTween, and no luck. Is there something wrong with the way I'm calling it from the Start function?

 private var moveTime;
 private var moveDistance;
 private var fadeTime;
 
 function Start()
 {
         moveTime = Random.Range(3,4);
         moveDistance = Random.Range(.1,.3);
         fadeTime = Random.Range(1,4);
         moveDown();
         fadeOut();
 }
 
 function moveDown()
 {
     
     iTween.MoveBy(gameObject, {"amount" : Vector3(0,-1*moveDistance,0), "time": moveTime, "easetype" : "easeInOutQuad", "oncomplete" : "moveUp", "oncompletetarget" : gameObject});
 }
 
 function moveUp()
 {
     iTween.MoveBy(gameObject, {"amount" : Vector3(0,moveDistance,0), "time": moveTime, "easetype" : "easeInOutQuad", "oncomplete" : "moveDown", "oncompletetarget" : gameObject});
 }
 
 function fadeOut()
 {
     iTween.FadeTo(gameObject, {"alpha" : 0, "time": fadeTime, "easetype" : "easeInOutQuad", "oncomplete" : "fadeIn"});
 }
 
 function fadeIn()
 {
     iTween.FadeTo(gameObject, {"alpha" : 1, "time": fadeTime, "easetype" : "easeInOutQuad", "oncomplete" : "fadeOut"});
 }