diff --git a/Optimisation_basic/include/CGAL/Optimisation/Access_coordinates_begin_d.h b/Optimisation_basic/include/CGAL/Optimisation/Access_coordinates_begin_d.h
index 4c8a48b7c73..3343bc18e2e 100644
--- a/Optimisation_basic/include/CGAL/Optimisation/Access_coordinates_begin_d.h
+++ b/Optimisation_basic/include/CGAL/Optimisation/Access_coordinates_begin_d.h
@@ -27,8 +27,8 @@
 
 CGAL_BEGIN_NAMESPACE
 
-struct Cartesian_tag;
-struct Homogeneous_tag;
+class Cartesian_tag;
+class Homogeneous_tag;
 
 // Class declaration
 // =================