--- a/src/ai/api/ai_engine.cpp	Tue Apr 15 22:18:11 2008 +0000
+++ b/src/ai/api/ai_engine.cpp	Tue Apr 15 22:29:00 2008 +0000
@@ -38,7 +38,7 @@
 
 /* static */ CargoID AIEngine::GetCargoType(EngineID engine_id)
 {
-	if (!IsValidEngine(engine_id)) return 0;
+	if (!IsValidEngine(engine_id)) return -1;
 
 	switch (::GetEngine(engine_id)->type) {
 		case VEH_ROAD: {
@@ -74,9 +74,9 @@
 	return ::CanRefitTo(engine_id, cargo_id);
 }
 
-/* static */ uint32 AIEngine::GetCapacity(EngineID engine_id)
+/* static */ int32 AIEngine::GetCapacity(EngineID engine_id)
 {
-	if (!IsValidEngine(engine_id)) return 0;
+	if (!IsValidEngine(engine_id)) return -1;
 
 	switch (::GetEngine(engine_id)->type) {
 		case VEH_ROAD:
@@ -86,7 +86,7 @@
 				if (capacities[c] == 0) continue;
 				return capacities[c];
 			}
-			return 0;
+			return -1;
 		} break;
 
 		case VEH_SHIP: {
@@ -103,16 +103,16 @@
 	}
 }
 
-/* static */ uint32 AIEngine::GetReliability(EngineID engine_id)
+/* static */ int32 AIEngine::GetReliability(EngineID engine_id)
 {
-	if (!IsValidEngine(engine_id)) return 0;
+	if (!IsValidEngine(engine_id)) return -1;
 
 	return (::GetEngine(engine_id)->reliability * 100 >> 16);
 }
 
-/* static */ uint32 AIEngine::GetMaxSpeed(EngineID engine_id)
+/* static */ int32 AIEngine::GetMaxSpeed(EngineID engine_id)
 {
-	if (!IsValidEngine(engine_id)) return 0;
+	if (!IsValidEngine(engine_id)) return -1;
 
 	switch (::GetEngine(engine_id)->type) {
 		case VEH_ROAD: {
@@ -141,9 +141,9 @@
 	}
 }
 
-/* static */ uint32 AIEngine::GetPrice(EngineID engine_id)
+/* static */ int32 AIEngine::GetPrice(EngineID engine_id)
 {
-	if (!IsValidEngine(engine_id)) return 0;
+	if (!IsValidEngine(engine_id)) return -1;
 
 	switch (::GetEngine(engine_id)->type) {
 		case VEH_ROAD: {
@@ -170,16 +170,16 @@
 	}
 }
 
-/* static */ uint32 AIEngine::GetMaxAge(EngineID engine_id)
+/* static */ int32 AIEngine::GetMaxAge(EngineID engine_id)
 {
-	if (!IsValidEngine(engine_id)) return 0;
+	if (!IsValidEngine(engine_id)) return -1;
 
 	return ::GetEngine(engine_id)->lifelength * 366;
 }
 
 /* static */ Money AIEngine::GetRunningCost(EngineID engine_id)
 {
-	if (!IsValidEngine(engine_id)) return 0;
+	if (!IsValidEngine(engine_id)) return -1;
 
 	/* We need to create an instance in order to obtain GetRunningCost.
 	 *  This means we temporary allocate a vehicle in the pool, but