Inventory -Argument is out of range - Diagnostics to be done

Hi im trying to create an inventory and when adding a new slot and an item i get the Argument is out of range error. also when i try and add 2 items at the same time only the first one goes through.

 void Update(){
         if (Input.GetKeyDown (KeyCode.Space)) {
             if (inTrigger = true) {
                 if (TriggerTag.Equals ("AppleTree")) {
                     Collect ("Apple", "An Apple", 1, null, Item.ItemType.Consumeable);
                 }
             } else {
                 Debug.Log ("There is nothing to collect!");
             }
         }
     }

  void Collect(string itemName,string itemDescription,int itemAmount,   Sprite itemIcon, Item.ItemType type){
             this.GetComponent<Inventory> ().AddItem (itemName,itemDescription,itemAmount,itemIcon,type);
         }

The above passes in some information you need for an item to be added and now to the next script

 public void AddItem (string itemName,string itemDescription,int itemAmount,   Sprite itemIcon, Item.ItemType type){
         if (inventory.Count <= InventoryMaxSize) {
             if (inventory.Count == 0) {
                 inventory.Add (new Item (itemName, itemDescription, itemAmount, itemIcon, type));
                 AddSlot ();
                 UpdateGui();
             } else {
                 for (int i = 0; i < inventory.Count; i++) {
                     if (inventory [i].ItemName == itemName) {
                         inventory [i].ItemAmount = inventory [i].ItemAmount + itemAmount;
                         UpdateGui();
                         break;
                     } else {
                         AddSlot();
                         inventory.Add (new Item (itemName, itemDescription, itemAmount, itemIcon, type));
                         UpdateGui ();
                         break;
                     }
                 }
             }
         } 
     }
 
     public void AddSlot(){
             GameObject slot = (GameObject)Instantiate (SlotGO);
             Slots.Add (slot);
             slot.transform.parent = GameObject.FindGameObjectWithTag ("InvGUI").transform;
             slot.GetComponent<RectTransform> ().localPosition = new Vector3 (SlotStartPosX, SlotStartPosY, 0);
             slot.GetComponent<RectTransform> ().localScale = new Vector3 (1, 1, 1);
             SlotStartPosY = SlotStartPosY - SlotMoveDown;
     }
     public void UpdateGui(){
         for (int i = 0; i < Slots.Count; i++) {
             for (int x = 0; i < inventory.Count; x++) {
                 Slots [i].GetComponent<SlotScript> ().item.ItemName = inventory [x].ItemName;
                 Slots [i].GetComponent<SlotScript> ().item.ItemDescription = inventory [x].ItemDescription;
                 Slots [i].GetComponent<SlotScript> ().item.ItemAmount = inventory [x].ItemAmount;
                 Slots [i].GetComponent<SlotScript> ().item.ItemIcon = inventory [x].ItemIcon;
                 Slots [i].GetComponent<SlotScript> ().item.itemType = inventory [x].itemType;
             }
         }
     }

The error points to the UpdateGUI function. also if i call the collect function twice from the first script with different item names it only seams to add the first one. any help would be greatly appreciated