diff --git a/libs/gtkmm2ext/idle_adjustment.cc b/libs/gtkmm2ext/idle_adjustment.cc
index 3e3a3da566..f74a3a9701 100644
--- a/libs/gtkmm2ext/idle_adjustment.cc
+++ b/libs/gtkmm2ext/idle_adjustment.cc
@@ -22,6 +22,8 @@
 #include <gtkmm/main.h>
 #include <iostream>
 
+#include "pbd/timersub.h"
+
 using namespace Gtk;
 using namespace sigc;
 using namespace Gtkmm2ext;