Why does this loop throw an error if int questComplete rises above 1?

Now I'm definitely still learning about how loops work so it's a bit of voodoo I have to throw into each code, but I can't for the life of me tease out why this script wouldn't work on two or even infinite quests. Little help?

 using UnityEngine;
 using System.Collections;
 
 public class completedQuests : MonoBehaviour {
 
     private questLogic mainScript;
     public baseQuest[] finishedQuests;
     public bool hasRun;
 
     private int incrementChecker;
     public int questsComplete;
     public int orderCompleted = 0; 
 
     // Use this for initialization
     void Start () {
     
         mainScript = GetComponent<questLogic>();
         incrementChecker = questsComplete;
 
     }
     
     // Update is called once per frame
     void Update () {
     
         if(questsComplete != incrementChecker){
 
             finishedQuests = new baseQuest[questsComplete];
 
             for(int i = 0; i < mainScript.allQuests.Length; i++){
 
                 if(mainScript.allQuests[i].currentStatus == mainScript.allQuests[i].questLength){
 
     error line==> finishedQuests[orderCompleted] = mainScript.allQuests[i];
                     orderCompleted++;
 
                 }
 
             }
 
             incrementChecker = questsComplete;
 
                     
         }
     }
 }