diff --git a/ttd.c b/ttd.c
--- a/ttd.c
+++ b/ttd.c
@@ -20,6 +20,7 @@
 #include "hal.h"
 #include "airport.h"
 #include "saveload.h"
+#include "ai.h"
 
 #include <stdarg.h>
 
@@ -429,6 +430,7 @@ void SetDebugString(const char *s)
 		_debug_spritecache_level = v;
 		_debug_misc_level = v;
 		_debug_grf_level = v;
+		_debug_ai_level = v;
 	}
 
 	// individual levels
@@ -445,6 +447,7 @@ void SetDebugString(const char *s)
 		if IS_LVL("misc") p = &_debug_misc_level;
 		else if IS_LVL("spritecache") p = &_debug_spritecache_level;
 		else if IS_LVL("grf") p = &_debug_grf_level;
+		else if IS_LVL("ai") p = &_debug_ai_level;
 		else {
 			ShowInfoF("Unknown debug level '%.*s'", s-t, t);
 			return;