--- a/src/ai/api/ai_engine.cpp	Tue Apr 15 22:34:49 2008 +0000
+++ b/src/ai/api/ai_engine.cpp	Tue Apr 15 22:38:05 2008 +0000
@@ -38,7 +38,7 @@
 
 /* static */ CargoID AIEngine::GetCargoType(EngineID engine_id)
 {
-	if (!IsValidEngine(engine_id)) return -1;
+	if (!IsValidEngine(engine_id)) return CT_INVALID;
 
 	switch (::GetEngine(engine_id)->type) {
 		case VEH_ROAD: {