diff --git a/Boolean_set_operations_2/include/CGAL/Polygon_set_2.h b/Boolean_set_operations_2/include/CGAL/Polygon_set_2.h
index d986acfd860..d1c2305c2dd 100644
--- a/Boolean_set_operations_2/include/CGAL/Polygon_set_2.h
+++ b/Boolean_set_operations_2/include/CGAL/Polygon_set_2.h
@@ -58,7 +58,7 @@ public:
   {}
 
   /*! Constructor with traits object. */
-  Polygon_set_2 (Traits_2& tr) :
+  Polygon_set_2 (const Traits_2& tr) :
     Base(tr)
   {}