diff --git a/Arrangement_on_surface_2/include/CGAL/Arr_unb_planar_topology_traits_2.h b/Arrangement_on_surface_2/include/CGAL/Arr_unb_planar_topology_traits_2.h
index 292389e4713..620bb2c7e25 100644
--- a/Arrangement_on_surface_2/include/CGAL/Arr_unb_planar_topology_traits_2.h
+++ b/Arrangement_on_surface_2/include/CGAL/Arr_unb_planar_topology_traits_2.h
@@ -554,8 +554,8 @@ public:
   */
   const Face* reference_face() const
   {
-    CGAL_error_msg("not implemented.");
-    return NULL;
+    assert(v_tr->halfedge()->direction() == ARR_LEFT_TO_RIGHT);
+    return v_tr->halfedge()->outer_ccb()->face();
   }
  
   //! reference_face (non-const version).
@@ -566,8 +566,8 @@ public:
   */
   Face* reference_face()
   {
-    CGAL_error_msg("not implemented.");
-    return NULL;
+    assert(v_tr->halfedge()->direction() == ARR_LEFT_TO_RIGHT);
+    return v_tr->halfedge()->outer_ccb()->face();
   }
 
   //@}