diff --git a/tile.h b/tile.h
index bcb06d29d4..6943f4a0e4 100644
--- a/tile.h
+++ b/tile.h
@@ -57,8 +57,9 @@ static inline bool IsTileType(TileIndex tile, TileType type)
 	return GetTileType(tile) == type;
 }
 
-static inline bool GetTileOwner(TileIndex tile)
+static inline Owner GetTileOwner(TileIndex tile)
 {
+	assert(tile < MapSize());
 	return _map_owner[tile];
 }