cs(14,21): error CS0029: Cannot implicitly convert type 'UnityEngine.Material[*,*]' to 'UnityEngine.Material[]'

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
 
public class Fractal : MonoBehaviour
{
    public Mesh mesh;
    public Material material;
    public int maxDepth;
    private int depth;
    private Material[] materials;
    private void InitializeMaterials()
    {
        materials = new Material [maxDepth + 1,2];
        for (int i = 0; i <= maxDepth; i++)
        {
            float t = i / (maxDepth - 1f);
            t *= t;
            materials[i] = new Material(material);
            materials[i].color = Color.Lerp(Color.white, Color.yellow, t);
        }
        materials[maxDepth].color = Color.magenta;
    }
    private void Start()
    {
        if (materials == null)
        {
            InitializeMaterials();
        }
        gameObject.AddComponent<MeshFilter>().mesh = mesh;
        gameObject.AddComponent<MeshRenderer>().material = materials[depth];       
        if (depth < maxDepth)
        {
            StartCoroutine(CreateChildren());
        }
    }
    private static Vector3[] childDirections = { Vector3.up, Vector3.right, Vector3.left, Vector3.forward, Vector3.back };
 
    private static Quaternion[] childOrientations = { Quaternion.identity, Quaternion.Euler(0f, 0f, -90f), Quaternion.Euler(0f, 0f, 90f), Quaternion.Euler(90f, 0f, 0f), Quaternion.Euler(-90f, 0f, 0f) };
 
    private IEnumerator CreateChildren()
    {
        for (int i = 0; i < childDirections.Length; i++)
        {
            yield return new WaitForSeconds(Random.Range(0.1f,0.5f));
            new GameObject("Fractal Child").AddComponent<Fractal>().Initialize(this, i);
        }
    }
    public float childScale;
    private void Initialize(Fractal parent, int childIndex)
    {
        mesh = parent.mesh;
        materials = parent.materials;
        material = parent.material;
        maxDepth = parent.maxDepth;
        depth = parent.depth + 1;
        childScale = parent.childScale;
        transform.parent = parent.transform;
        transform.localScale = Vector3.one * childScale;
        transform.localPosition = childDirections[childIndex] * (0.5f + 0.5f * childScale);
        transform.localRotation = childOrientations[childIndex];
    }

private Material[,] materials;