diff --git a/newgrf_engine.c b/newgrf_engine.c
--- a/newgrf_engine.c
+++ b/newgrf_engine.c
@@ -765,7 +765,7 @@ bool UsesWagonOverride(const Vehicle* v)
  * @param vehicle  The vehicle to evaluate the callback for, or NULL if it doesnt exist yet
  * @return The value the callback returned, or CALLBACK_FAILED if it failed
  */
-uint16 GetVehicleCallback(byte callback, uint32 param1, uint32 param2, EngineID engine, const Vehicle *v)
+uint16 GetVehicleCallback(uint16 callback, uint32 param1, uint32 param2, EngineID engine, const Vehicle *v)
 {
 	const SpriteGroup *group;
 	ResolverObject object;
diff --git a/newgrf_spritegroup.h b/newgrf_spritegroup.h
--- a/newgrf_spritegroup.h
+++ b/newgrf_spritegroup.h
@@ -156,7 +156,7 @@ void InitializeSpriteGroupPool(void);
 
 
 typedef struct ResolverObject {
-	byte callback;
+	uint16 callback;
 	uint32 callback_param1;
 	uint32 callback_param2;