diff --git a/src/slic3r/GUI/wxExtensions.hpp b/src/slic3r/GUI/wxExtensions.hpp
index 69369b3ff..e40306ea1 100644
--- a/src/slic3r/GUI/wxExtensions.hpp
+++ b/src/slic3r/GUI/wxExtensions.hpp
@@ -31,6 +31,7 @@ wxMenuItem* append_submenu(wxMenu* menu, wxMenu* sub_menu, int id, const wxStrin
 wxMenuItem* append_menu_radio_item(wxMenu* menu, int id, const wxString& string, const wxString& description, 
     std::function<void(wxCommandEvent& event)> cb, wxEvtHandler* event_handler);
 
+class wxDialog;
 void    msw_buttons_rescale(wxDialog* dlg, const int em_unit, const std::vector<int>& btn_ids);
 int     em_unit(wxWindow* win);