NullReferenceException error

Getting a NullReferenceException error (Object reference not set to an instance of an object) at the line that starts "transform.rotation".

 var targetTag:String;
 var speed:float;
 var acceleration:float;
 var lifeDuration:float = 3.0;
 var maxRotation:float;  
 
 function Update()
 {
     if(lifeDuration <= 0)
         Destroy(gameObject);
     else
     {
         var targets: GameObject[] = GameObject.FindGameObjectsWithTag("Enemy");
        var closest: GameObject;
         var closestDist = Mathf.Infinity;
 
         for(Target in targets)
         {
             var dist = (transform.position - Target.transform.position).sqrMagnitude;
 
             if(dist < closestDist)
             {
                 closestDist = dist;
                 closest = Target;
             }
         }
         //transform.LookAt(closest.transform);
 
         transform.rotation = Quaternion.Slerp(transform.rotation, Quaternion.LookRotation(closest.transform.position - transform.position), maxRotation * Time.deltaTime);
 
         speed += Time.deltaTime * acceleration;
         transform.position += transform.forward * speed * Time.deltaTime;        
 
         lifeDuration -= Time.deltaTime;
     }
 }