diff --git a/configure.ac b/configure.ac
index 23afdd70..8579ff95 100644
--- a/configure.ac
+++ b/configure.ac
@@ -159,7 +159,7 @@ AC_ARG_ENABLE(debug,
       CFLAGS="$nxflags -O0 -g3"
     ])
     ENABLE_CWFLAGS=yes
-    CPPFLAGS="$CPPFLAGS -DDEBUG=1 -UFORTIFY_SOURCE"
+    CPPFLAGS="$CPPFLAGS -DDEBUG=1 -U_FORTIFY_SOURCE"
   ])
 ])