error CS0122: `GameController.AddScore(int)' is inaccessible due to its protection level

Hello everyone. I'm trying to develop a script that add points when the player touch an object. I'm following the official unity tutorial but it seems to have problems. When I'm in the unity platform I always have this error:

  error CS0122: `GameController.AddScore(int)' is inaccessible due to its protection level

Here is the two scripts used:

DestroyByContacts (applied to object that need to be destroyed after collision and add point:

 using UnityEngine;
 using System.Collections;
 
 public class DestroyByContact : MonoBehaviour {
 
     public int scoreValue;
     private GameController gameController;
 
     void Start() {
         GameObject gameControllerObject = GameObject.FindWithTag ("GameController");
         if (gameControllerObject != null) {
             gameController = gameControllerObject.GetComponent<GameController>();
         }
         if (gameController == null) {
             Debug.Log("Cannot find 'GameController' script");
         }
     }
 
     void OnCollisionEnter2D(Collision2D coll)
     {
         if (coll.gameObject.tag == "Player")
         {
             Destroy(this.gameObject);
             gameController.AddScore(scoreValue);
         }
 
     }
 }

GameController ( applied to the GameController Object of the scene:

 using UnityEngine;
 using System.Collections;
 
 public class GameController : MonoBehaviour {
 
     public GUIText scoreText;
     private int score;
 
     void Start ()
     {
         score = 0;
         UpdateScore ();
     }
 
     void UpdateScore() 
     {
         scoreText.text = "Score: " + score;
     }
 
     void AddScore( int newScoreValue )
     {
         score += newScoreValue;
         UpdateScore ();
     }
 }

Please help! I'm becoming crazy! I don't know how to adjust this error.