diff --git a/Surface_mesh/include/CGAL/Surface_mesh/Surface_mesh.h b/Surface_mesh/include/CGAL/Surface_mesh/Surface_mesh.h
index 63da3e80d90..c4a561ad0e1 100644
--- a/Surface_mesh/include/CGAL/Surface_mesh/Surface_mesh.h
+++ b/Surface_mesh/include/CGAL/Surface_mesh/Surface_mesh.h
@@ -198,7 +198,7 @@ namespace CGAL {
 
         SM_Edge_index() : halfedge_(-1) { }
 
-        SM_Edge_index(size_type idx) : halfedge_(idx * 2) { }
+        explicit SM_Edge_index(size_type idx) : halfedge_(idx * 2) { }
 
         explicit SM_Edge_index(SM_Halfedge_index he) : halfedge_(he) { }