diff --git a/chrome/content/zotero/xpcom/data/itemFields.js b/chrome/content/zotero/xpcom/data/itemFields.js
index 9dcb0983b..67bb7053f 100644
--- a/chrome/content/zotero/xpcom/data/itemFields.js
+++ b/chrome/content/zotero/xpcom/data/itemFields.js
@@ -204,7 +204,7 @@ Zotero.ItemFields = new function() {
 			throw new Error("Item type field data not found for itemTypeID " + itemTypeID);
 		}
 		
-		return _itemTypeFields[itemTypeID];
+		return [..._itemTypeFields[itemTypeID]];
 	}
 	
 	
@@ -317,7 +317,7 @@ Zotero.ItemFields = new function() {
 		}
 		
 		return _typeFieldIDsByBase[baseFieldID] ?
-			_typeFieldIDsByBase[baseFieldID] : false;
+			[..._typeFieldIDsByBase[baseFieldID]] : false;
 	}