diff --git a/src/window.cpp b/src/window.cpp
index 24b2aae8fe..2dc00a3185 100644
--- a/src/window.cpp
+++ b/src/window.cpp
@@ -860,7 +860,7 @@ bool IsWindowOfPrototype(const Window *w, const Widget *widget)
  *       \c w->original_widget points to the original widgets,
  *       \c w->widget_count contains number of widgets in the allocated memory.
  */
-void AssignWidgetToWindow(Window *w, const Widget *widget)
+static void AssignWidgetToWindow(Window *w, const Widget *widget)
 {
 	w->original_widget = widget;
 
diff --git a/src/window_gui.h b/src/window_gui.h
index a20c005f75..5872c68092 100644
--- a/src/window_gui.h
+++ b/src/window_gui.h
@@ -652,7 +652,6 @@ Window *BringWindowToFrontById(WindowClass cls, WindowNumber number);
 Window *FindWindowFromPt(int x, int y);
 
 bool IsWindowOfPrototype(const Window *w, const Widget *widget);
-void AssignWidgetToWindow(Window *w, const Widget *widget);
 
 /**
  * Open a new window.