diff --git a/ai/ai.c b/ai/ai.c
--- a/ai/ai.c
+++ b/ai/ai.c
@@ -77,7 +77,7 @@ void AI_PutCommandInQueue(byte player, u
  */
 int32 AI_DoCommand(uint tile, uint32 p1, uint32 p2, uint32 flags, uint procc)
 {
-	byte old_lp;
+	PlayerID old_lp;
 	int32 res = 0;
 
 	/* If you enable DC_EXEC with DC_QUERY_COST you are a really strange
@@ -118,7 +118,7 @@ int32 AI_DoCommand(uint tile, uint32 p1,
 /**
  * Run 1 tick of the AI. Don't overdo it, keep it realistic.
  */
-static void AI_RunTick(byte player)
+static void AI_RunTick(PlayerID player)
 {
 	extern void AiNewDoGameLoop(Player *p);
 
@@ -181,7 +181,7 @@ void AI_RunGameLoop(void)
 /**
  * A new AI sees the day of light. You can do here what ever you think is needed.
  */
-void AI_StartNewAI(byte player)
+void AI_StartNewAI(PlayerID player)
 {
 	/* Called if a new AI is booted */
 	_ai_player[player].active = true;
@@ -190,7 +190,7 @@ void AI_StartNewAI(byte player)
 /**
  * This AI player died. Give it some chance to make a final puf.
  */
-void AI_PlayerDied(byte player)
+void AI_PlayerDied(PlayerID player)
 {
 	/* Called if this AI died */
 	_ai_player[player].active = false;
diff --git a/ai/ai.h b/ai/ai.h
--- a/ai/ai.h
+++ b/ai/ai.h
@@ -37,8 +37,8 @@ VARDEF AIStruct _ai;
 VARDEF AIPlayer _ai_player[MAX_PLAYERS];
 
 // ai.c
-void AI_StartNewAI(byte player);
-void AI_PlayerDied(byte player);
+void AI_StartNewAI(PlayerID player);
+void AI_PlayerDied(PlayerID player);
 void AI_RunGameLoop(void);
 void AI_Initialize(void);
 void AI_Uninitialize(void);
diff --git a/ai/default/default.c b/ai/default/default.c
--- a/ai/default/default.c
+++ b/ai/default/default.c
@@ -3183,14 +3183,15 @@ static void AiStateBuildRoadVehicles(Pla
 	const AiDefaultBlockData *ptr;
 	TileIndex tile;
 	uint loco_id;
-	int veh, i;
+	EngineID veh;
+	int i;
 
 	ptr = _road_default_block_data[p->ai.src.cur_building_rule]->data;
 	for(;ptr->mode != 0;ptr++) {}
 	tile = TILE_ADD(p->ai.src.use_tile, ToTileIndexDiff(ptr->tileoffs));
 
 	veh = AiChooseRoadVehToBuild(p->ai.cargo_type, p->player_money, tile);
-	if (veh == -1) {
+	if (veh == INVALID_ENGINE) {
 		p->ai.state = AIS_0;
 		return;
 	}