diff --git a/functions.h b/functions.h
--- a/functions.h
+++ b/functions.h
@@ -203,7 +203,7 @@ void InvalidateWindowClasses(byte cls);
 void DeleteWindowById(WindowClass cls, WindowNumber number);
 
 void SetObjectToPlaceWnd(int icon, byte mode, Window *w);
-void SetObjectToPlace(int icon, byte mode, byte window_class, uint16 window_num);
+void SetObjectToPlace(int icon, byte mode, WindowClass window_class, WindowNumber window_num);
 
 void ResetObjectToPlace();
 
diff --git a/viewport.c b/viewport.c
--- a/viewport.c
+++ b/viewport.c
@@ -2060,7 +2060,7 @@ void SetObjectToPlaceWnd(int icon, byte 
 
 #include "table/animcursors.h"
 
-void SetObjectToPlace(int icon, byte mode, byte window_class, uint16 window_num)
+void SetObjectToPlace(int icon, byte mode, WindowClass window_class, WindowNumber window_num)
 {
 	TileHighlightData *thd = _thd_ptr;
 	Window *w;
diff --git a/viewport.h b/viewport.h
--- a/viewport.h
+++ b/viewport.h
@@ -86,9 +86,9 @@ typedef struct TileHighlightData {
 	byte next_drawstyle;
 
 	byte place_mode;
-	byte window_class;
 	bool make_square_red;
-	uint16 window_number;
+	WindowClass window_class;
+	WindowNumber window_number;
 
 	int userdata;
 	TileIndex redsq;