diff --git a/meson.build b/meson.build
index 6ccbb95..50d7809 100644
--- a/meson.build
+++ b/meson.build
@@ -2,7 +2,7 @@ project('tio', 'c',
     version : '1.33',
     license : [ 'GPL-2'],
     meson_version : '>= 0.53.2',
-    default_options : [ 'warning_level=2', 'buildtype=release', 'c_std=gnu17' ]
+    default_options : [ 'warning_level=2', 'buildtype=release', 'c_std=gnu99' ]
 )
 
 # The tag date of the project_version(), update when the version bumps.