diff --git a/chrome/content/zotero/xpcom/rdf/serialize.js b/chrome/content/zotero/xpcom/rdf/serialize.js
index bc506c6887..c8953b943d 100644
--- a/chrome/content/zotero/xpcom/rdf/serialize.js
+++ b/chrome/content/zotero/xpcom/rdf/serialize.js
@@ -111,8 +111,8 @@ __Serializer.prototype.makeUpPrefix = function(uri) {
 //  - do stuff about the docu first and then (or first) about its primary topic.
 
 __Serializer.prototype.rootSubjects = function(sts) {
-    var incoming = [];
-    var subjects = [];
+    var incoming = {};
+    var subjects = {};
     var sz = this;
     var allBnodes = {};