IndexOutOfRangeException: Index was outside the bounds of the array error in unity

Question

So I was following this tutorial(https://catlikecoding.com/unity/tutorials/hex-map/part-1/). Eventually, I reached this part of the tutorial:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.EventSystems;
public class HexMapEditor : MonoBehaviour
{
    public Color[] colors;

    public HexGrid hexGrid;

    private Color activeColor;
    
    public void selectColor (int index) {
        activeColor = colors[index];
        Debug.Log("**Color changed to : " + colors[index]);
    }
    void handleInput () {
    Ray inputRay = Camera.main.ScreenPointToRay(Input.mousePosition);
    RaycastHit hit;
        if (Physics.Raycast(inputRay, out hit)) {
            hexGrid.colorCell(hit.point, activeColor);
            
        }

    }

    
    
    void Awake(){

        selectColor(0);
    }

    // Start is called before the first frame update
    void Start()
    {
        
    }

    // Update is called once per frame
    void Update()
    {
        if (Input.GetMouseButtonDown(0) &&
            !EventSystem.current.IsPointerOverGameObject())
        {
            handleInput();
        }
    }
}

using UnityEngine;
using UnityEngine.UI;
using System;

public class HexGrid : MonoBehaviour
{
---
public void colorCell (Vector3 position, Color color) {
        position = transform.InverseTransformPoint(position);
        HexCoordinates coordinates = HexCoordinates.FromPosition(position);
        int index = coordinates.X + coordinates.Z * width + coordinates.Z / 2;
        HexCell cell = cells[index];
        cell.color = color;
        hexMesh.Triangulate(cells);
    }
---
}

For some reason, this causes an "IndexOutOfRangeException: Index was outside the bounds of the array" error.

However, something like this works:

using UnityEngine;
using UnityEngine.UI;
using System;

public class HexGrid : MonoBehaviour
{
---
public void colorCell (Vector3 position) {
        position = transform.InverseTransformPoint(position);
        HexCoordinates coordinates = HexCoordinates.FromPosition(position);
        int index = coordinates.X + coordinates.Z * width + coordinates.Z / 2;
        HexCell cell = cells[index];
        cell.color = touchedColor;
        hexMesh.Triangulate(cells);
    }
void handleInput () {
    Ray inputRay = Camera.main.ScreenPointToRay(Input.mousePosition);
    RaycastHit hit;
        if (Physics.Raycast(inputRay, out hit)) {
            colorCell(hit.point);
            
        }

    }
---
}

Is there a way to solve this? Thanks in advance and also sorry if this problem is caused by a stupid oversight.