diff --git a/lcl/interfaces/gtk3/gtk3lclintf.inc b/lcl/interfaces/gtk3/gtk3lclintf.inc
index 259e838c92..bbaa7f19c2 100644
--- a/lcl/interfaces/gtk3/gtk3lclintf.inc
+++ b/lcl/interfaces/gtk3/gtk3lclintf.inc
@@ -1305,7 +1305,7 @@ function TGtk3WidgetSet.IsDesignerDC(WindowHandle: HWND; DC: HDC): Boolean;
 begin
   Result := IsValidHandle(WindowHandle) and (TGtk3Widget(WindowHandle) is TGtk3DesignWidget);
   if Result then
-    Result := TGtk3DesignWidget(DC).DesignContext = DC;
+    Result := TGtk3DesignWidget(WindowHandle).DesignContext = DC;
 end;
 
 function TGtk3WidgetSet.ReleaseDesignerDC(Window: HWND; DC: HDC): Integer;