Script error HELP

So i keep getting a weird error that i don't have it set or something Error is:NullReferenceException: Object reference not set to an instance of an object Boo.Lang.Runtime.RuntimeServices.InvokeBinaryOperator (System.String operatorName, System.Object lhs, System.Object rhs) Water Physics.Update () (at Assets/Scripts/Water Physics.js:13) I have no idea what it means my code:

 //deathnadosharkgames
 var waterLevel : float;
 var floatHeight : float;
 var bouncedamp : float;
 var offtheset : Vector3;
 
 private var forcefactor : float;
 private var actionpoint : Vector3;
 private var uplift : Vector3;
 
 function Update () {
 
     actionpoint = transform.postion + transform.TransformDirection(offtheset);
     forcefactor = 1f - ((actionpoint.y - waterLevel) / floatHeight);
     if(forcefactor > 0f) {
     
     
     uplift = -Physics.gravity *(forcefactor - rigidbody.velocity.y * bouncedamp);
     rigidbody.AddForceAtPosition(uplift, actionpoint);
     
     
     }
     
 
 }