diff --git a/src/void_cmd.cpp b/src/void_cmd.cpp
--- a/src/void_cmd.cpp
+++ b/src/void_cmd.cpp
@@ -23,7 +23,7 @@ static void DrawTile_Void(TileInfo *ti)
 }
 
 
-static uint GetSlopePixelZ_Void(TileIndex tile, uint x, uint y)
+static int GetSlopePixelZ_Void(TileIndex tile, uint x, uint y)
 {
 	return TilePixelHeight(tile);
 }
@@ -60,7 +60,7 @@ static TrackStatus GetTileTrackStatus_Vo
 	return 0;
 }
 
-static CommandCost TerraformTile_Void(TileIndex tile, DoCommandFlag flags, uint z_new, Slope tileh_new)
+static CommandCost TerraformTile_Void(TileIndex tile, DoCommandFlag flags, int z_new, Slope tileh_new)
 {
 	return_cmd_error(STR_ERROR_OFF_EDGE_OF_MAP);
 }