UNITYEXPECTION: ToString is not allowed to be called during serialization, call it from OnEnable instead.

I am making a inventory system in unity, when I add Items to this dictionary I get this error after I add the second Item to it.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 [CreateAssetMenu(fileName = "New Item Database", menuName = "Inventory Syste/Items/Database")]
 public class Itemdatatbase : ScriptableObject, ISerializationCallbackReceiver 
 {
   public ItemObject[] Items;
   public Dictionary<ItemObject, int> GetId = new Dictionary<ItemObject, int>();
   
   
 
   public void OnAfterDeserialize()
   {
     GetId = new Dictionary<ItemObject, int>();
     for (int i = 0; i < Items.Length; i++)
     {
       Debug.Log("added");
       GetId.Add(Items[i], i);
     }
   }
 
   public void OnBeforeSerialize()
   {
     
   }
 
 }