diff --git a/src/ai/api/ai_airport.cpp b/src/ai/api/ai_airport.cpp
--- a/src/ai/api/ai_airport.cpp
+++ b/src/ai/api/ai_airport.cpp
@@ -12,7 +12,7 @@
 
 /* static */ bool AIAirport::IsValidAirportType(AirportType type)
 {
-	return type >= 0 && type < NUM_AIRPORTS && ::GetAirport(type)->IsAvailable();
+	return type >= 0 && type < (AirportType)NUM_AIRPORTS && ::GetAirport(type)->IsAvailable();
 }
 
 /* static */ Money AIAirport::GetPrice(AirportType type)