diff --git a/Number_types/include/CGAL/CORE_Expr.h b/Number_types/include/CGAL/CORE_Expr.h
index 3ddb4c7f78fd..f38681a08e24 100644
--- a/Number_types/include/CGAL/CORE_Expr.h
+++ b/Number_types/include/CGAL/CORE_Expr.h
@@ -135,7 +135,7 @@ template <> class Algebraic_structure_traits< CORE::Expr >
           if((inf == 1) && (sup == 1)){
             return true;
           }
-          return x.cmp(Type::getOne());
+          return x.cmp(Type::getOne()) == 0;
         }
     };