ArgumentOutOfRange error with list

I'm getting an argument out of range error and I don't know how to fix it. Here is my relevant code:

 /// Max allowed slots in inventory.
 /// </summary>
 public int maxInventorySpace = 5;


 /// <summary>
 /// Equips the item located at the associated index.
 /// </summary>
 public int currentlyEquippedItemIndex = 0;


 /// <summary>
 /// Items currently in inventory.
 /// </summary>
 public List<ItemPickUp> itemsInInventory = new List<ItemPickUp>();

   
 /// <summary>
 /// Scroll through the items with the mouse wheel.
 /// </summary>
 void ScrollItems()
 {
     if (Input.GetAxis("Mouse ScrollWheel") > 0f)
     {
         if (currentlyEquippedItemIndex >= itemsInInventory.Count - 1)
             currentlyEquippedItemIndex = 0;
         else
             currentlyEquippedItemIndex++;
     }

     else if (Input.GetAxis("Mouse ScrollWheel") < 0f)
     {
         if (currentlyEquippedItemIndex <= 0)
             currentlyEquippedItemIndex = itemsInInventory.Count - 1;
         else
             currentlyEquippedItemIndex--;
     }

     // Select the item at the index if it is not already equipped
     if (itemsInInventory[currentlyEquippedItemIndex].isEquipped == false)
     {
         SelectItem(currentlyEquippedItemIndex);
     }
 }

Unity says I'm getting the error because of this:

 // Select the item at the index if it is not already equipped
         if (itemsInInventory[currentlyEquippedItemIndex].isEquipped == false)
         {
             SelectItem(currentlyEquippedItemIndex);
         }