diff --git a/src/ai/api/ai_bridge.cpp b/src/ai/api/ai_bridge.cpp
index 4e7d51776e..5f28ff51c8 100644
--- a/src/ai/api/ai_bridge.cpp
+++ b/src/ai/api/ai_bridge.cpp
@@ -25,7 +25,7 @@
 
 /* static */ BridgeID AIBridge::GetBridgeID(TileIndex tile)
 {
-	if (!IsBridgeTile(tile)) return -1;
+	if (!IsBridgeTile(tile)) return (BridgeID)-1;
 	return (BridgeID)::GetBridgeType(tile);
 }