Getting an error trying to assign a value to an array of classes element

When i try to assign a value to an element of the array of classes i get the "Object reference not set to an instance of an object" error.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class Test : MonoBehaviour
 {
     public TestClass[,] TestArray;
 
     void Start()
     {
         TestArray[0, 0] = new TestClass();
     }
 }
 
 public class TestClass
 {

 }