diff --git a/src/airport_gui.cpp b/src/airport_gui.cpp
index f114dfa211..51b332da17 100644
--- a/src/airport_gui.cpp
+++ b/src/airport_gui.cpp
@@ -233,7 +233,7 @@ class BuildAirportWindow : public PickerWindowBase {
 	{
 		DropDownList list;
 
-		for (uint i = 0; i < AirportClass::GetClassCount(); i++) {
+		for (uint i = 0; AirportClass::IsClassIDValid((AirportClassID)i); i++) {
 			list.emplace_back(new DropDownListStringItem(AirportClass::Get((AirportClassID)i)->name, i, false));
 		}
 
@@ -313,7 +313,7 @@ public:
 		switch (widget) {
 			case WID_AP_CLASS_DROPDOWN: {
 				Dimension d = {0, 0};
-				for (uint i = 0; i < AirportClass::GetClassCount(); i++) {
+				for (uint i = 0; AirportClass::IsClassIDValid((AirportClassID)i); i++) {
 					SetDParam(0, AirportClass::Get((AirportClassID)i)->name);
 					d = maxdim(d, GetStringBoundingBox(STR_BLACK_STRING));
 				}
diff --git a/src/newgrf_class.h b/src/newgrf_class.h
index e0efeb4ae4..7c7afc1dc6 100644
--- a/src/newgrf_class.h
+++ b/src/newgrf_class.h
@@ -59,6 +59,7 @@ public:
 	static uint GetUIClassCount();
 	static bool HasUIClass();
 	static Tid GetUIClass(uint index);
+	static bool IsClassIDValid(Tid cls_id);
 	static NewGRFClass *Get(Tid cls_id);
 
 	static const Tspec *GetByGrf(uint32 grfid, byte local_id, int *index);
diff --git a/src/newgrf_class_func.h b/src/newgrf_class_func.h
index 33c6a15894..5b35f8e84e 100644
--- a/src/newgrf_class_func.h
+++ b/src/newgrf_class_func.h
@@ -95,6 +95,15 @@ DEFINE_NEWGRF_CLASS_METHOD(void)::Assign(Tspec *spec)
 	Get(spec->cls_id)->Insert(spec);
 }
 
+/**
+ * Get whether the class ID is valid (for iteration).
+ * @return Whether the class ID is valid.
+ */
+DEFINE_NEWGRF_CLASS_METHOD(bool)::IsClassIDValid(Tid cls_id)
+{
+	return cls_id < Tmax && classes[cls_id].global_id != 0;
+}
+
 /**
  * Get a particular class.
  * @param cls_id The id for the class.
@@ -232,6 +241,7 @@ DEFINE_NEWGRF_CLASS_METHOD(const Tspec *)::GetByGrf(uint32 grfid, byte local_id,
 	template Tid name::Allocate(uint32 global_id); \
 	template void name::Insert(Tspec *spec); \
 	template void name::Assign(Tspec *spec); \
+	template bool name::IsClassIDValid(Tid cls_id); \
 	template NewGRFClass<Tspec, Tid, Tmax> *name::Get(Tid cls_id); \
 	template uint name::GetClassCount(); \
 	template uint name::GetUIClassCount(); \
diff --git a/src/newgrf_roadstop.cpp b/src/newgrf_roadstop.cpp
index 275f9ab988..ed79ecdf1d 100644
--- a/src/newgrf_roadstop.cpp
+++ b/src/newgrf_roadstop.cpp
@@ -501,7 +501,7 @@ void TriggerRoadStopRandomisation(Station *st, TileIndex tile, RoadStopRandomTri
 bool GetIfNewStopsByType(RoadStopType rs, RoadType roadtype)
 {
 	if (!(RoadStopClass::GetClassCount() > 1 || RoadStopClass::Get(ROADSTOP_CLASS_DFLT)->GetSpecCount() > 1)) return false;
-	for (uint i = 0; i < RoadStopClass::GetClassCount(); i++) {
+	for (uint i = 0; RoadStopClass::IsClassIDValid((RoadStopClassID)i); i++) {
 		// We don't want to check the default or waypoint classes. These classes are always available.
 		if (i == ROADSTOP_CLASS_DFLT || i == ROADSTOP_CLASS_WAYP) continue;
 		RoadStopClass *roadstopclass = RoadStopClass::Get((RoadStopClassID)i);
diff --git a/src/object_gui.cpp b/src/object_gui.cpp
index c76184add1..b4ac303dba 100644
--- a/src/object_gui.cpp
+++ b/src/object_gui.cpp
@@ -153,7 +153,7 @@ public:
 
 		this->object_classes.clear();
 
-		for (uint i = 0; i < ObjectClass::GetClassCount(); i++) {
+		for (uint i = 0; ObjectClass::IsClassIDValid((ObjectClassID)i); i++) {
 			ObjectClass *objclass = ObjectClass::Get((ObjectClassID)i);
 			if (objclass->GetUISpecCount() == 0) continue; // Is this needed here?
 			object_classes.push_back((ObjectClassID)i);
@@ -181,7 +181,7 @@ public:
 			/* Check if the previously selected object class is not available anymore as a
 			 * result of starting a new game without the corresponding NewGRF. */
 			bool available = false;
-			for (uint i = 0; ObjectClass::GetClassCount(); ++i) {
+			for (uint i = 0; ObjectClass::IsClassIDValid((ObjectClassID)i); ++i) {
 				if ((ObjectClassID)i == _selected_object_class) {
 					available = true;
 					break;
diff --git a/src/rail_gui.cpp b/src/rail_gui.cpp
index 871328615f..96a65d36c9 100644
--- a/src/rail_gui.cpp
+++ b/src/rail_gui.cpp
@@ -1202,7 +1202,7 @@ public:
 
 		this->station_classes.clear();
 
-		for (uint i = 0; i < StationClass::GetClassCount(); i++) {
+		for (uint i = 0; StationClass::IsClassIDValid((StationClassID)i); i++) {
 			StationClassID station_class_id = (StationClassID)i;
 			if (station_class_id == StationClassID::STAT_CLASS_WAYP) {
 				// Skip waypoints.
@@ -1236,7 +1236,7 @@ public:
 			/* Check if the previously selected station class is not available anymore as a
 			 * result of starting a new game without the corresponding NewGRF. */
 			bool available = false;
-			for (uint i = 0; i < StationClass::GetClassCount(); ++i) {
+			for (uint i = 0; StationClass::IsClassIDValid((StationClassID)i); ++i) {
 				if ((StationClassID)i == _railstation.station_class) {
 					available = true;
 					break;
diff --git a/src/road_gui.cpp b/src/road_gui.cpp
index 741478d028..acff565d1d 100644
--- a/src/road_gui.cpp
+++ b/src/road_gui.cpp
@@ -1374,7 +1374,7 @@ public:
 
 		this->roadstop_classes.clear();
 
-		for (uint i = 0; i < RoadStopClass::GetClassCount(); i++) {
+		for (uint i = 0; RoadStopClass::IsClassIDValid((RoadStopClassID)i); i++) {
 			RoadStopClassID rs_id = (RoadStopClassID)i;
 			if (rs_id == ROADSTOP_CLASS_WAYP) {
 				// Skip waypoints.