Changing the image of a button with a script

Hi, I was unable to run a script to change the image of a button.

Script: (C#)

using UnityEngine;
using System.Collections;
using UnityEngine.UI;
using System;

public class Bouton123 : MonoBehaviour {
	public Button button;
	public Sprite Image1;
	void Start () {
		button = GetComponent<Button>();
		button.image.sprite = Image1;
	void Update () {

Error: “NullReferenceException: Object reference not set to an instance of an object”

Someone can help me ?

you should say :

button.GetComponent<Image>().sprite = Image1;