diff --git a/src/water_cmd.cpp b/src/water_cmd.cpp
--- a/src/water_cmd.cpp
+++ b/src/water_cmd.cpp
@@ -263,8 +263,6 @@ static CommandCost DoBuildLock(TileIndex
 	cost.AddCost(ret);
 
 	/* lower tile */
-	WaterClass wc_lower = IsWaterTile(tile - delta) ? GetWaterClass(tile - delta) : WATER_CLASS_CANAL;
-
 	if (!IsWaterTile(tile - delta)) {
 		ret = DoCommand(tile - delta, 0, 0, flags, CMD_LANDSCAPE_CLEAR);
 		if (ret.Failed()) return ret;
@@ -274,10 +272,9 @@ static CommandCost DoBuildLock(TileIndex
 	if (!IsTileFlat(tile - delta)) {
 		return_cmd_error(STR_ERROR_LAND_SLOPED_IN_WRONG_DIRECTION);
 	}
+	WaterClass wc_lower = IsWaterTile(tile - delta) ? GetWaterClass(tile - delta) : WATER_CLASS_CANAL;
 
 	/* upper tile */
-	WaterClass wc_upper = IsWaterTile(tile + delta) ? GetWaterClass(tile + delta) : WATER_CLASS_CANAL;
-
 	if (!IsWaterTile(tile + delta)) {
 		ret = DoCommand(tile + delta, 0, 0, flags, CMD_LANDSCAPE_CLEAR);
 		if (ret.Failed()) return ret;
@@ -287,6 +284,7 @@ static CommandCost DoBuildLock(TileIndex
 	if (!IsTileFlat(tile + delta)) {
 		return_cmd_error(STR_ERROR_LAND_SLOPED_IN_WRONG_DIRECTION);
 	}
+	WaterClass wc_upper = IsWaterTile(tile + delta) ? GetWaterClass(tile + delta) : WATER_CLASS_CANAL;
 
 	if (IsBridgeAbove(tile) || IsBridgeAbove(tile - delta) || IsBridgeAbove(tile + delta)) {
 		return_cmd_error(STR_ERROR_MUST_DEMOLISH_BRIDGE_FIRST);