diff --git a/arangod/VocBase/Traverser.h b/arangod/VocBase/Traverser.h
index bf9a90cb6b..ad346bb206 100644
--- a/arangod/VocBase/Traverser.h
+++ b/arangod/VocBase/Traverser.h
@@ -120,8 +120,8 @@ namespace triagens {
             }
           }
 
-          void toJson (triagens::basics::Json& json,
-                       TRI_memory_zone_t* zone) const;
+          virtual void toJson (triagens::basics::Json& json,
+                               TRI_memory_zone_t* zone) const;
 
           bool matchesCheck (TRI_doc_mptr_t& element,
                              TRI_document_collection_t* collection,