Делал по курсу. Там всё работало. Object reference not set to an instance of an object. Unity. С#

using UnityEngine;  
using System.Collections;  
using UnityEngine.UI;  
using UnityEngine.SceneManagement;  
  
public class Buttons : MonoBehaviour {  
  
    public Sprite button, pressed;  
    public bool music;  
      
    private Image img;  
  
    void Start () {  
        img = GetComponent <Image> ();  
    }  
  
    void OnMouseDown () {  
        img.sprite = pressed; // Тут ошибка  
    }  
  
    void OnMouseUp () {  
        img.sprite = button; // И тут  
  
    }  
  
}