diff --git a/src/script/api/script_airport.cpp b/src/script/api/script_airport.cpp
--- a/src/script/api/script_airport.cpp
+++ b/src/script/api/script_airport.cpp
@@ -158,7 +158,7 @@
 
 /* static */ uint16 ScriptAirport::GetMaintenanceCostFactor(AirportType type)
 {
-	if (!IsAirportInformationAvailable(type)) return INVALID_TOWN;
+	if (!IsAirportInformationAvailable(type)) return 0;
 
 	return AirportSpec::Get(type)->maintenance_cost;
 }
diff --git a/src/script/api/script_road.cpp b/src/script/api/script_road.cpp
--- a/src/script/api/script_road.cpp
+++ b/src/script/api/script_road.cpp
@@ -452,8 +452,8 @@ static bool NeighbourHasReachableRoad(::
 
 /* static */ int32 ScriptRoad::GetNeighbourRoadCount(TileIndex tile)
 {
-	if (!::IsValidTile(tile)) return false;
-	if (!IsRoadTypeAvailable(GetCurrentRoadType())) return false;
+	if (!::IsValidTile(tile)) return -1;
+	if (!IsRoadTypeAvailable(GetCurrentRoadType())) return -1;
 
 	::RoadType rt = (::RoadType)GetCurrentRoadType();
 	int32 neighbour = 0;
@@ -627,7 +627,7 @@ static bool NeighbourHasReachableRoad(::
 
 /* static */ int32 ScriptRoad::GetMaxSpeed(RoadType road_type)
 {
-	if (!ScriptRoad::IsRoadTypeAvailable(road_type)) return 0;
+	if (!ScriptRoad::IsRoadTypeAvailable(road_type)) return -1;
 
 	return GetRoadTypeInfo((::RoadType)road_type)->max_speed;
 }