NullReferenceException problems.

Hello, Unity Answers. I'm having trouble with a NullReferenceException. The error reads : Object reference not set to an instance of an object. I know what this means. What I don't know is why/how I am getting the error. I've done some further research on my own but I haven't found anything practical to my issue (I'm working on a Tower Defense game if that helps).
My code is below:

 #pragma strict
 
 //BulletHit will move towards an enemy. If it hits the enemy,
 //the enemy will recieve the bullets damage.
 //Declaring variables ::
 var bulletDmg : int;
 var bulletSpeed : float = 1;
 var despawnTime : float = 2;
 private var bulletRB : Rigidbody2D;
 
 //Runs on start.
 function Start ()
 {
     bulletRB = GetComponent.<Rigidbody2D>();
     Despawn();
 }
 
 //Adds a force to the bullet in the direction of the enemy.
 function MoveToEnemy (towerPos : Vector3, enemyPos : Vector3) {
     //forceDir is equal to the Vector3 between the enemy & tower
     var forceDir : Vector3 = GetEnemyDir(towerPos, enemyPos);
     bulletRB.AddForce(forceDir * bulletSpeed);          //ERROR HERE
 }

Note: towerPos and enemyPos are given to MoveToEnemy from the class BasicTower. I've debugged BasicTower to insure that the sent items are not null.
function GetEnemyDir:
Returns a given enemy Vector3 converted to coordinates relative to the tower.