diff --git a/src/gui/Dialog.hxx b/src/gui/Dialog.hxx
index eccf2e93d..63d6b2698 100644
--- a/src/gui/Dialog.hxx
+++ b/src/gui/Dialog.hxx
@@ -73,7 +73,7 @@ class Dialog : public GuiObject
     void addCancelWidget(Widget* w) { _cancelWidget = w; }
     void setFocus(Widget* w);
 
-    FBSurface& surface() { return *_surface; }
+    FBSurface& surface() const { return *_surface; }
 
   protected:
     virtual void draw();