diff --git a/meson.build b/meson.build
index 641208158c..b1b21a8fa6 100644
--- a/meson.build
+++ b/meson.build
@@ -66,7 +66,7 @@ if debug
     gtk_debug_cflags += '-DG_ENABLE_CONSISTENCY_CHECKS'
   endif
 elif optimization in ['2', '3', 's']
-  gtk_debug_cflags += '-DG_DISABLE_CAST_CHECKS -DG_DISABLE_ASSERT'
+  gtk_debug_cflags += ['-DG_DISABLE_CAST_CHECKS', '-DG_DISABLE_ASSERT']
 endif
 
 add_project_arguments(gtk_debug_cflags, language: 'c')