diff --git a/src/order_cmd.cpp b/src/order_cmd.cpp
--- a/src/order_cmd.cpp
+++ b/src/order_cmd.cpp
@@ -567,6 +567,7 @@ CommandCost CmdInsertOrder(TileIndex til
 
 	if (sel_ord > v->GetNumOrders()) return CMD_ERROR;
 
+	if (v->GetNumOrders() >= MAX_VEH_ORDER_ID) return_cmd_error(STR_8832_TOO_MANY_ORDERS);
 	if (!Order::CanAllocateItem()) return_cmd_error(STR_8831_NO_MORE_SPACE_FOR_ORDERS);
 	if (v->orders.list == NULL && !OrderList::CanAllocateItem()) return_cmd_error(STR_8831_NO_MORE_SPACE_FOR_ORDERS);
 
diff --git a/src/order_type.h b/src/order_type.h
--- a/src/order_type.h
+++ b/src/order_type.h
@@ -12,10 +12,12 @@ typedef uint16 OrderID;
 typedef uint16 OrderListID;
 typedef uint16 DestinationID;
 
-enum {
-	INVALID_VEH_ORDER_ID = 0xFF,
-};
+/** Invalid vehicle order index (sentinel) */
+static const VehicleOrderID INVALID_VEH_ORDER_ID = 0xFF;
+/** Last valid VehicleOrderID. */
+static const VehicleOrderID MAX_VEH_ORDER_ID     = INVALID_VEH_ORDER_ID - 1;
 
+/** Invalid order (sentinel) */
 static const OrderID INVALID_ORDER = 0xFFFF;
 
 /* Order types */