diff --git a/Property_map/include/CGAL/property_map.h b/Property_map/include/CGAL/property_map.h
index 988c7ab6583..d57c53310e8 100644
--- a/Property_map/include/CGAL/property_map.h
+++ b/Property_map/include/CGAL/property_map.h
@@ -473,7 +473,7 @@ struct Constant_property_map
 
   typedef KeyType                                       key_type;
   typedef ValueType                                     value_type;
-  typedef value_type&                                   reference;
+  typedef const value_type&                             reference;
   typedef boost::read_write_property_map_tag            category;
 
   Constant_property_map(const value_type& default_value = value_type()) : default_value (default_value) { }