diff --git a/ChangeLog b/ChangeLog
index 2660bcea4..4fe0c5fce 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,5 +1,7 @@
 2004-08-09  Alexandre Duret-Lutz  <adl@src.lip6.fr>
 
+	* m4/ndebug.m4: Update CPPFLAGS, not CFLAGS.
+
 	* src/tgbaalgos/ltl2tgba_fm.hh (ltl_to_tgba_fm): Document all
 	parameters.
 	* src/tgbaalgos/ltl2tgba_lacim.hh (ltl_to_tgba_lacim): Likewise.
diff --git a/m4/ndebug.m4 b/m4/ndebug.m4
index 2e716a0a5..076f40aac 100644
--- a/m4/ndebug.m4
+++ b/m4/ndebug.m4
@@ -1,6 +1,6 @@
 AC_DEFUN([adl_NDEBUG],
  [AC_ARG_ENABLE([assert],
-  [AC_HELP_STRING([--enable-assert],[turn on assertions])])
+  [AC_HELP_STRING([--enable-assert], [turn on assertions])])
   if test "$enable_assert" != yes; then
-    CFLAGS="$CFLAGS -DNDEBUG"
+    CPPFLAGS="$CPPFLAGS -DNDEBUG"
   fi])