diff --git a/gtk/gtkmain.c b/gtk/gtkmain.c
index 12b5d6659d..ca56f94a38 100644
--- a/gtk/gtkmain.c
+++ b/gtk/gtkmain.c
@@ -657,16 +657,18 @@ do_pre_parse_initialization (int    *argc,
   GDK_PRIVATE_CALL (gdk_pre_parse) ();
   gdk_event_handler_set ((GdkEventFunc)gtk_main_do_event, NULL, NULL);
 
-#ifdef G_ENABLE_DEBUG
   env_string = g_getenv ("GTK_DEBUG");
   if (env_string != NULL)
     {
+#ifdef G_ENABLE_DEBUG
       debug_flags[0].flags = g_parse_debug_string (env_string,
                                                    gtk_debug_keys,
                                                    G_N_ELEMENTS (gtk_debug_keys));
+#else
+      g_warning ("GTK_DEBUG set but ignored because gtk isn't built with G_ENABLE_DEBUG");
+#endif  /* G_ENABLE_DEBUG */
       env_string = NULL;
     }
-#endif  /* G_ENABLE_DEBUG */
 
   env_string = g_getenv ("GTK3_MODULES");
   if (env_string)