diff --git a/src/water_cmd.cpp b/src/water_cmd.cpp
--- a/src/water_cmd.cpp
+++ b/src/water_cmd.cpp
@@ -73,7 +73,7 @@ static const uint8 _flood_from_dirs[] = 
  */
 static inline void MarkTileDirtyIfCanalOrRiver(TileIndex tile)
 {
-	if (IsTileType(tile, MP_WATER) && (IsCanal(tile) || IsRiver(tile))) MarkTileDirtyByTile(tile);
+	if (IsValidTile(tile) && IsTileType(tile, MP_WATER) && (IsCanal(tile) || IsRiver(tile))) MarkTileDirtyByTile(tile);
 }
 
 /**