Help with raycast!!

I need to set a script that when i click on the Enemy gameObject appears, in top of the screen , the life and the name of the Enemy.... Now i' am using the raycast:

 void Update () {
         if (Input.GetMouseButtonDown (0)) {
             Ray ray = Camera.main.ScreenPointToRay (Input.mousePosition);
             RaycastHit hit;
             if (Physics.Raycast (ray, out hit, 100)) {
                 if (hit.collider.gameObject.tag == "Enemy")
                     print ("enemy hit");
             }
         }
     }

But this error appears every time i click :

NullReferenceException: Object reference not set to an instance of an object EnemyManager.Update () (at Assets/Scripts/EnemyManager.cs:14)