diff --git a/src/window.cpp b/src/window.cpp
--- a/src/window.cpp
+++ b/src/window.cpp
@@ -134,7 +134,7 @@ void Window::RaiseButtons()
 	}
 }
 
-void Window::InvalidateWidget(byte widget_index)
+void Window::InvalidateWidget(byte widget_index) const
 {
 	const Widget *wi = &this->widget[widget_index];
 
diff --git a/src/window.h b/src/window.h
--- a/src/window.h
+++ b/src/window.h
@@ -297,7 +297,7 @@ struct Window {
 	void CDECL SetWidgetsDisabledState(bool disab_stat, int widgets, ...);
 	void CDECL SetWidgetsHiddenState(bool hidden_stat, int widgets, ...);
 	void CDECL SetWidgetsLoweredState(bool lowered_stat, int widgets, ...);
-	void InvalidateWidget(byte widget_index);
+	void InvalidateWidget(byte widget_index) const;
 };
 
 struct querystr_d {