diff --git a/src/viewport.cpp b/src/viewport.cpp
--- a/src/viewport.cpp
+++ b/src/viewport.cpp
@@ -488,7 +488,7 @@ Point GetTileZoomCenterWindow(bool in, W
  * @param widget_zoom_in widget index for window with zoom-in button
  * @param widget_zoom_out widget index for window with zoom-out button
  */
-void HandleZoomMessage(Window *w, const Viewport *vp, byte widget_zoom_in, byte widget_zoom_out)
+void HandleZoomMessage(Window *w, const Viewport *vp, WidgetID widget_zoom_in, WidgetID widget_zoom_out)
 {
 	w->SetWidgetDisabledState(widget_zoom_in, vp->zoom <= _settings_client.gui.zoom_min);
 	w->SetWidgetDirty(widget_zoom_in);