diff --git a/src/departures.cpp b/src/departures.cpp
index ffc6570527..5a8cd23b37 100644
--- a/src/departures.cpp
+++ b/src/departures.cpp
@@ -106,6 +106,7 @@ static inline bool VehicleSetNextDepartureTime(DateTicks *previous_departure, ui
 
 					/* Find next available slots */
 					for (auto current_offset : v->orders.list->GetScheduledDispatch()) {
+						if (current_offset >= dispatch_duration) continue;
 						DateTicksScaled current_departure = begin_time + current_offset;
 						while (current_departure <= earliest_departure) {
 							current_departure += dispatch_duration;
diff --git a/src/lang/english.txt b/src/lang/english.txt
index c52fae2259..c1aac3e0c4 100644
--- a/src/lang/english.txt
+++ b/src/lang/english.txt
@@ -6224,6 +6224,8 @@ STR_SCHDISPATCH_SUMMARY_L2                                      :{BLACK}This sch
 STR_SCHDISPATCH_SUMMARY_L3                                      :{BLACK}Maximum delay of {STRING3} is allowed before the slot is skipped.
 STR_SCHDISPATCH_SUMMARY_NOT_ENABLED                             :{BLACK}This schedule is not active.
 
+STR_SCHDISPATCH_SLOT_OUTSIDE_SCHEDULE                           :{BLACK}One or more departure slots are outside the schedule duration.
+
 # Modifier key toggle window
 STR_MODIFIER_KEY_TOGGLE_CAPTION                                 :{WHITE}Modifier keys
 STR_SHIFT_KEY_NAME                                              :{BLACK}Shift
diff --git a/src/schdispatch_gui.cpp b/src/schdispatch_gui.cpp
index 0d55797259..14b45b3ee5 100644
--- a/src/schdispatch_gui.cpp
+++ b/src/schdispatch_gui.cpp
@@ -127,10 +127,12 @@ static int CalculateMaxRequiredVehicle(Ticks timetable_duration, uint32 schedule
 	std::vector<std::pair<uint32, int>> indices;
 	for (int i = 0; i < required_loop; i++) {
 		for (uint32 offset : offsets) {
+			if (offset >= schedule_duration) continue;
 			indices.push_back(std::make_pair(i * schedule_duration + offset, 1));
 			indices.push_back(std::make_pair(i * schedule_duration + offset + timetable_duration, -1));
 		}
 	}
+	if (indices.empty()) return -1;
 	std::sort(indices.begin(), indices.end());
 	int current_count = 0;
 	int vehicle_count = 0;
@@ -149,6 +151,7 @@ struct SchdispatchWindow : Window {
 
 	uint item_count = 0;     ///< Number of scheduled item
 	bool last_departure_future; ///< True if last departure is currently displayed in the future
+	uint warning_count = 0;
 
 	SchdispatchWindow(WindowDesc *desc, WindowNumber window_number) :
 			Window(desc),
@@ -201,6 +204,10 @@ struct SchdispatchWindow : Window {
 
 			case WID_SCHDISPATCH_SUMMARY_PANEL:
 				size->height = WD_FRAMERECT_TOP + 4 * FONT_HEIGHT_NORMAL + WD_FRAMERECT_BOTTOM;
+				if (warning_count > 0) {
+					const Dimension warning_dimensions = GetSpriteSize(SPR_WARNING_SIGN);
+					size->height += warning_count * max<int>(warning_dimensions.height, FONT_HEIGHT_NORMAL);
+				}
 				break;
 		}
 	}
@@ -373,6 +380,39 @@ struct SchdispatchWindow : Window {
 
 					SetTimetableParams(0, v->orders.list->GetScheduledDispatchDelay());
 					DrawString(r.left + WD_FRAMERECT_LEFT, r.right - WD_FRAMERECT_RIGHT, y, STR_SCHDISPATCH_SUMMARY_L3);
+					y += FONT_HEIGHT_NORMAL;
+
+					uint warnings = 0;
+					auto draw_warning = [&](StringID text) {
+						const Dimension warning_dimensions = GetSpriteSize(SPR_WARNING_SIGN);
+						int step_height = max<int>(warning_dimensions.height, FONT_HEIGHT_NORMAL);
+						int left = r.left + WD_FRAMERECT_LEFT;
+						int right = r.right - WD_FRAMERECT_RIGHT;
+						const bool rtl = (_current_text_dir == TD_RTL);
+						DrawSprite(SPR_WARNING_SIGN, 0, rtl ? right - warning_dimensions.width - 5 : left + 5, y + (step_height - warning_dimensions.height) / 2);
+						if (rtl) {
+							right -= (warning_dimensions.width + 10);
+						} else {
+							left += (warning_dimensions.width + 10);
+						}
+						DrawString(left, right, y + (step_height - FONT_HEIGHT_NORMAL) / 2, text);
+						y += step_height;
+						warnings++;
+					};
+
+					uint32 duration = v->orders.list->GetScheduledDispatchDuration();
+					for (uint32 slot : v->orders.list->GetScheduledDispatch()) {
+						if (slot >= duration) {
+							draw_warning(STR_SCHDISPATCH_SLOT_OUTSIDE_SCHEDULE);
+							break;
+						}
+					}
+
+					if (warnings != this->warning_count) {
+						SchdispatchWindow *mutable_this = const_cast<SchdispatchWindow *>(this);
+						mutable_this->warning_count = warnings;
+						mutable_this->ReInit();
+					}
 				}
 
 				break;
diff --git a/src/timetable_cmd.cpp b/src/timetable_cmd.cpp
index 983003c76a..d61cecdb80 100644
--- a/src/timetable_cmd.cpp
+++ b/src/timetable_cmd.cpp
@@ -737,6 +737,7 @@ static DateTicksScaled GetScheduledDispatchTime(Vehicle *v, int wait_offset)
 
 	/* Find next available slots */
 	for (auto current_offset : v->orders.list->GetScheduledDispatch()) {
+		if (current_offset >= dispatch_duration) continue;
 		if (int32(current_offset) <= last_dispatched_offset) {
 			current_offset += dispatch_duration * ((last_dispatched_offset + dispatch_duration - current_offset) / dispatch_duration);
 		}