diff --git a/debug.h b/debug.h
--- a/debug.h
+++ b/debug.h
@@ -9,6 +9,7 @@
 	#define DEBUG(name, level) if (level == 0 || _debug_ ## name ## _level >= level) debug
 
 	extern int _debug_ai_level;
+	extern int _debug_driver_level;
 	extern int _debug_grf_level;
 	extern int _debug_map_level;
 	extern int _debug_misc_level;