--- a/services/sync/modules/engines/bookmarks.js
+++ b/services/sync/modules/engines/bookmarks.js
@@ -410,17 +410,19 @@ BookmarksEngine.prototype = {
 
     // Don't bother finding a dupe if the incoming item has duplicates.
     if (item.hasDupe) {
       this._log.trace(item.id + " already a dupe: not finding one.");
       return;
     }
     let mapped = this._mapDupe(item);
     this._log.debug(item.id + " mapped to " + mapped);
-    return mapped;
+    // We must return a string, not an object, and the entries in the GUIDMap
+    // are created via "new String()" making them an object.
+    return mapped ? mapped.toString() : mapped;
   }
 };
 
 function BookmarksStore(name, engine) {
   Store.call(this, name, engine);
 
   // Explicitly nullify our references to our cached services so we don't leak
   Svc.Obs.add("places-shutdown", function() {