diff --git a/src/schdispatch_cmd.cpp b/src/schdispatch_cmd.cpp
index 536dccd0b0..40bb201978 100644
--- a/src/schdispatch_cmd.cpp
+++ b/src/schdispatch_cmd.cpp
@@ -155,7 +155,7 @@ CommandCost CmdScheduledDispatchSetDuration(TileIndex tile, DoCommandFlag flags,
 	uint schedule_index = GB(p1, 20, 12);
 
 	Vehicle *v = Vehicle::GetIfValid(veh);
-	if (v == nullptr || !v->IsPrimaryVehicle()) return CMD_ERROR;
+	if (v == nullptr || !v->IsPrimaryVehicle() || p2 == 0) return CMD_ERROR;
 
 	CommandCost ret = CheckOwnership(v->owner);
 	if (ret.Failed()) return ret;
@@ -330,7 +330,7 @@ CommandCost CmdScheduledDispatchAddNewSchedule(TileIndex tile, DoCommandFlag fla
 	VehicleID veh = GB(p1, 0, 20);
 
 	Vehicle *v = Vehicle::GetIfValid(veh);
-	if (v == nullptr || !v->IsPrimaryVehicle()) return CMD_ERROR;
+	if (v == nullptr || !v->IsPrimaryVehicle() || p2 == 0) return CMD_ERROR;
 
 	CommandCost ret = CheckOwnership(v->owner);
 	if (ret.Failed()) return ret;