diff --git a/src/ship_cmd.cpp b/src/ship_cmd.cpp
--- a/src/ship_cmd.cpp
+++ b/src/ship_cmd.cpp
@@ -882,19 +882,16 @@ CommandCost CmdSellShip(TileIndex tile, 
  */
 CommandCost CmdStartStopShip(TileIndex tile, uint32 flags, uint32 p1, uint32 p2)
 {
-	Vehicle *v;
-	uint16 callback;
-
 	if (!IsValidVehicleID(p1)) return CMD_ERROR;
 
-	v = GetVehicle(p1);
+	Vehicle *v = GetVehicle(p1);
 
 	if (v->type != VEH_SHIP || !CheckOwnership(v->owner)) return CMD_ERROR;
 
 	/* Check if this ship can be started/stopped. The callback will fail or
 	 * return 0xFF if it can. */
-	callback = GetVehicleCallback(CBID_VEHICLE_START_STOP_CHECK, 0, 0, v->engine_type, v);
-	if (callback != CALLBACK_FAILED && callback != 0xFF) {
+	uint16 callback = GetVehicleCallback(CBID_VEHICLE_START_STOP_CHECK, 0, 0, v->engine_type, v);
+	if (callback != CALLBACK_FAILED && GB(callback, 0, 8) != 0xFF) {
 		StringID error = GetGRFStringID(GetEngineGRFID(v->engine_type), 0xD000 + callback);
 		return_cmd_error(error);
 	}