diff --git a/spot/mc/unionfind.cc b/spot/mc/unionfind.cc
index 99c4a81f1..de4921055 100644
--- a/spot/mc/unionfind.cc
+++ b/spot/mc/unionfind.cc
@@ -1,5 +1,5 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2015, 2016, 2018 Laboratoire de Recherche et
+// Copyright (C) 2015, 2016, 2018, 2021 Laboratoire de Recherche et
 // Developpement de l'Epita
 //
 // This file is part of Spot, a model checking library.
@@ -20,7 +20,7 @@
 
 #include "config.h"
 #include <spot/mc/unionfind.hh>
-#include <assert.h>
+#include <cassert>
 
 namespace spot
 {
diff --git a/spot/twacube/cube.cc b/spot/twacube/cube.cc
index b76c08b2b..0293288dd 100644
--- a/spot/twacube/cube.cc
+++ b/spot/twacube/cube.cc
@@ -1,5 +1,5 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2015, 2016, 2018, 2020 Laboratoire de Recherche et
+// Copyright (C) 2015, 2016, 2018, 2020, 2021 Laboratoire de Recherche et
 // Developpement de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
@@ -21,7 +21,7 @@
 #include <sstream>
 #include <iostream>
 #include <spot/twacube/cube.hh>
-#include <assert.h>
+#include <cassert>
 
 namespace spot
 {
diff --git a/spot/twacube_algos/convert.cc b/spot/twacube_algos/convert.cc
index e8878b890..bdbca1ca9 100644
--- a/spot/twacube_algos/convert.cc
+++ b/spot/twacube_algos/convert.cc
@@ -20,7 +20,7 @@
 #include "config.h"
 #include <spot/twacube_algos/convert.hh>
 #include <spot/twaalgos/contains.hh>
-#include <assert.h>
+#include <cassert>
 
 namespace spot
 {
diff --git a/tests/sanity/style.test b/tests/sanity/style.test
index 59c2f5c31..f181c747e 100755
--- a/tests/sanity/style.test
+++ b/tests/sanity/style.test
@@ -327,6 +327,8 @@ for dir in "$TOP/spot" "$TOP/bin" "$TOP/tests"; do
                   $GREP '#.*include.*[<"]config.h[">]' $tmp &&
                       diag 'Do not include config.h in header files.'
           esac
+	  $GREP '[<"]assert.h[">]' $tmp &&
+              diag 'include <cassert> in C++ files'
 	  ;;
 	*.cc)
 	  if $GREP 'namespace$' $tmp >/dev/null; then
@@ -346,7 +348,10 @@ for dir in "$TOP/spot" "$TOP/bin" "$TOP/tests"; do
                   else
                       diag "*.cc files should include "'"config.h"'
                   fi;;
-          esac;;
+          esac
+          $GREP '[<"]assert.h[">]' $tmp &&
+              diag 'include <cassert> in C++ files'
+          ;;
       esac
       case $file in
 	*/bin/*|*.cc);;