diff --git a/libs/gtkmm2ext/gtkmm2ext/gui_thread.h b/libs/gtkmm2ext/gtkmm2ext/gui_thread.h
index 63786ed90f..f473751118 100644
--- a/libs/gtkmm2ext/gtkmm2ext/gui_thread.h
+++ b/libs/gtkmm2ext/gtkmm2ext/gui_thread.h
@@ -26,10 +26,6 @@
 
 #include "gtkmm2ext/visibility.h"
 
-namespace sigc {
-	struct trackable;
-}
-
 #define ENSURE_GUI_THREAD(obj,method, ...) if (!Gtkmm2ext::UI::instance()->caller_is_self()) { abort (); }
 
 #define gui_context() Gtkmm2ext::UI::instance() /* a UICallback-derived object that specifies the event loop for GUI signal handling */
diff --git a/libs/gtkmm2ext/gtkmm2ext/visibility_tracker.h b/libs/gtkmm2ext/gtkmm2ext/visibility_tracker.h
index 541a1947ca..1e0aea7a4b 100644
--- a/libs/gtkmm2ext/gtkmm2ext/visibility_tracker.h
+++ b/libs/gtkmm2ext/gtkmm2ext/visibility_tracker.h
@@ -23,7 +23,7 @@
 
 #include "gtkmm2ext/visibility.h"
 
-namespace GTK {
+namespace Gtk {
 	class Window;
 }