diff --git a/src/object_cmd.cpp b/src/object_cmd.cpp
--- a/src/object_cmd.cpp
+++ b/src/object_cmd.cpp
@@ -381,7 +381,7 @@ static void DrawTile_Object(TileInfo *ti
 	if (spec->flags & OBJECT_FLAG_ALLOW_UNDER_BRIDGE) DrawBridgeMiddle(ti);
 }
 
-static uint GetSlopePixelZ_Object(TileIndex tile, uint x, uint y)
+static int GetSlopePixelZ_Object(TileIndex tile, uint x, uint y)
 {
 	if (IsOwnedLand(tile)) {
 		int z;
@@ -716,7 +716,7 @@ static void ChangeTileOwner_Object(TileI
 	}
 }
 
-static CommandCost TerraformTile_Object(TileIndex tile, DoCommandFlag flags, uint z_new, Slope tileh_new)
+static CommandCost TerraformTile_Object(TileIndex tile, DoCommandFlag flags, int z_new, Slope tileh_new)
 {
 	ObjectType type = GetObjectType(tile);