diff --git a/src/road_map.h b/src/road_map.h
index 01daba8e76..62f125dbb7 100644
--- a/src/road_map.h
+++ b/src/road_map.h
@@ -198,6 +198,7 @@ static inline bool HasTileRoadType(TileIndex t, RoadType rt)
  */
 static inline Owner GetRoadOwner(TileIndex t, RoadType rt)
 {
+	assert(IsTileType(t, MP_ROAD) || IsTileType(t, MP_STATION) || IsTileType(t, MP_TUNNELBRIDGE));
 	switch (rt) {
 		default: NOT_REACHED();
 		case ROADTYPE_ROAD: return (Owner)GB(IsNormalRoadTile(t) ? _m[t].m1 : _me[t].m7, 0, 5);