diff --git a/src/dock_gui.cpp b/src/dock_gui.cpp
--- a/src/dock_gui.cpp
+++ b/src/dock_gui.cpp
@@ -59,7 +59,7 @@ void CcBuildCanal(const CommandCost &res
  */
 static TileIndex GetOtherAqueductEnd(TileIndex tile_from, TileIndex *tile_to = NULL)
 {
-	uint z;
+	int z;
 	DiagDirection dir = GetInclinedSlopeDirection(GetTileSlope(tile_from, &z));
 
 	/* If the direction isn't right, just return the next tile so the command