diff --git a/src/order_cmd.cpp b/src/order_cmd.cpp
index ec1d96f38c..35b4407e50 100644
--- a/src/order_cmd.cpp
+++ b/src/order_cmd.cpp
@@ -3192,6 +3192,10 @@ bool EvaluateDispatchSlotConditionalOrder(const Order *order, const Vehicle *v,
 		offset = last % sched.GetScheduledDispatchDuration();
 	} else {
 		StateTicks slot = GetScheduledDispatchTime(sched, state_ticks);
+		if (slot == INVALID_STATE_TICKS) {
+			/* No next dispatch */
+			return OrderConditionCompare(order->GetConditionComparator(), 0, 0);
+		}
 		offset = (slot - sched.GetScheduledDispatchStartTick()).base() % sched.GetScheduledDispatchDuration();
 	}
 
diff --git a/src/schdispatch_gui.cpp b/src/schdispatch_gui.cpp
index 37e59d1b4e..1309cad045 100644
--- a/src/schdispatch_gui.cpp
+++ b/src/schdispatch_gui.cpp
@@ -508,7 +508,8 @@ struct SchdispatchWindow : GeneralVehicleWindow {
 					}
 
 					bool have_last = false;
-					if (ds.GetScheduledDispatchLastDispatch() % ds.GetScheduledDispatchDuration() == slot->offset) {
+					int32_t last_dispatch = ds.GetScheduledDispatchLastDispatch();
+					if (last_dispatch != INVALID_SCHEDULED_DISPATCH_OFFSET && (last_dispatch % ds.GetScheduledDispatchDuration() == slot->offset)) {
 						_temp_special_strings[0] += '\n';
 						_temp_special_strings[0] += GetString(STR_SCHDISPATCH_SLOT_TOOLTIP_LAST);
 						if (_settings_time.time_in_minutes) {
@@ -521,7 +522,7 @@ struct SchdispatchWindow : GeneralVehicleWindow {
 						have_last = true;
 					}
 					StateTicks next_slot = GetScheduledDispatchTime(ds, _state_ticks);
-					if ((next_slot - ds.GetScheduledDispatchStartTick()).AsTicks() % ds.GetScheduledDispatchDuration() == slot->offset) {
+					if (next_slot != INVALID_STATE_TICKS && ((next_slot - ds.GetScheduledDispatchStartTick()).AsTicks() % ds.GetScheduledDispatchDuration() == slot->offset)) {
 						if (!have_last) _temp_special_strings[0] += '\n';
 						_temp_special_strings[0] += GetString(STR_SCHDISPATCH_SLOT_TOOLTIP_NEXT);
 						if (_settings_time.time_in_minutes) {
@@ -631,9 +632,14 @@ struct SchdispatchWindow : GeneralVehicleWindow {
 				const StateTicks end_tick = ds.GetScheduledDispatchStartTick() + ds.GetScheduledDispatchDuration();
 
 				StateTicks slot = GetScheduledDispatchTime(ds, _state_ticks);
-				int32_t next_offset = (slot - ds.GetScheduledDispatchStartTick()).AsTicks() % ds.GetScheduledDispatchDuration();
+				int32_t next_offset = (slot != INVALID_STATE_TICKS) ? (slot - ds.GetScheduledDispatchStartTick()).AsTicks() % ds.GetScheduledDispatchDuration() : INT32_MIN;
 
-				int32_t last_dispatch = ds.GetScheduledDispatchLastDispatch() % ds.GetScheduledDispatchDuration();
+				int32_t last_dispatch;
+				if (ds.GetScheduledDispatchLastDispatch() != INVALID_SCHEDULED_DISPATCH_OFFSET) {
+					last_dispatch = ds.GetScheduledDispatchLastDispatch() % ds.GetScheduledDispatchDuration();
+				} else {
+					last_dispatch = INT32_MIN;
+				}
 
 				for (int y = r.top + 1; num < maxval; y += this->resize.step_height) { /* Draw the rows */
 					for (uint i = 0; i < this->num_columns && num < maxval; i++, num++) {
@@ -829,9 +835,11 @@ struct SchdispatchWindow : GeneralVehicleWindow {
 					}
 
 					const StateTicks next_departure = GetScheduledDispatchTime(ds, _state_ticks);
-					set_next_departure_update(next_departure + ds.GetScheduledDispatchDelay());
-					SetDParam(0, next_departure);
-					DrawString(ir.left, ir.right, y, STR_SCHDISPATCH_SUMMARY_NEXT_AVAILABLE_DEPARTURE);
+					if (next_departure != INVALID_STATE_TICKS) {
+						set_next_departure_update(next_departure + ds.GetScheduledDispatchDelay());
+						SetDParam(0, next_departure);
+						DrawString(ir.left, ir.right, y, STR_SCHDISPATCH_SUMMARY_NEXT_AVAILABLE_DEPARTURE);
+					}
 					y += GetCharacterHeight(FS_NORMAL);
 
 					departure_time_warnings(next_departure);
diff --git a/src/timetable_cmd.cpp b/src/timetable_cmd.cpp
index d42cc5e51d..6ef55fb2b5 100644
--- a/src/timetable_cmd.cpp
+++ b/src/timetable_cmd.cpp
@@ -804,6 +804,12 @@ void UpdateSeparationOrder(Vehicle *v_start)
 	}
 }
 
+/**
+ * Get next scheduled dispatch time
+ * @param ds Dispatch schedule.
+ * @param leave_time Leave time.
+ * @return Dispatch time, or INVALID_STATE_TICKS
+ */
 StateTicks GetScheduledDispatchTime(const DispatchSchedule &ds, StateTicks leave_time)
 {
 	const uint32_t dispatch_duration = ds.GetScheduledDispatchDuration();
@@ -821,7 +827,7 @@ StateTicks GetScheduledDispatchTime(const DispatchSchedule &ds, StateTicks leave
 		last_dispatched_offset = ds.GetScheduledDispatchLastDispatch();
 	}
 
-	StateTicks first_slot = -1;
+	StateTicks first_slot = INVALID_STATE_TICKS;
 
 	/* Find next available slots */
 	for (const DispatchSlot &slot : ds.GetScheduledDispatch()) {
@@ -839,7 +845,7 @@ StateTicks GetScheduledDispatchTime(const DispatchSchedule &ds, StateTicks leave
 			current_departure += dispatch_duration * ((minimum + dispatch_duration - current_departure - 1) / dispatch_duration);
 		}
 
-		if (first_slot == -1 || first_slot > current_departure) {
+		if (first_slot == INVALID_STATE_TICKS || first_slot > current_departure) {
 			first_slot = current_departure;
 		}
 	}
@@ -891,7 +897,7 @@ void UpdateVehicleTimetable(Vehicle *v, bool travelling)
 
 			const int wait_offset = real_current_order->GetTimetabledWait();
 			StateTicks slot = GetScheduledDispatchTime(ds, _state_ticks + wait_offset);
-			if (slot > -1) {
+			if (slot != INVALID_STATE_TICKS) {
 				just_started = !HasBit(v->vehicle_flags, VF_TIMETABLE_STARTED);
 				SetBit(v->vehicle_flags, VF_TIMETABLE_STARTED);
 				v->lateness_counter = (_state_ticks - slot + wait_offset).AsTicks();
diff --git a/src/timetable_gui.cpp b/src/timetable_gui.cpp
index 271982a91e..403cae1ff7 100644
--- a/src/timetable_gui.cpp
+++ b/src/timetable_gui.cpp
@@ -196,7 +196,7 @@ static void FillTimetableArrivalDepartureTable(const Vehicle *v, VehicleOrderID
 				predicted_ds.UpdateScheduledDispatchToDate(_state_ticks + sum);
 				StateTicks slot = GetScheduledDispatchTime(predicted_ds, _state_ticks + sum + order->GetTimetabledWait());
 				predicted_ds.ReturnSchedule(ds);
-				if (slot <= -1) return;
+				if (slot == INVALID_STATE_TICKS) return;
 				sum = (slot - _state_ticks).AsTicks();
 				predicted = true;
 				no_offset = true;