diff --git a/libs/ardour/ardour/rt_task.h b/libs/ardour/ardour/rt_task.h
index ec2a118c49..bfee21f2a9 100644
--- a/libs/ardour/ardour/rt_task.h
+++ b/libs/ardour/ardour/rt_task.h
@@ -19,6 +19,7 @@
 #ifndef _ardour_rt_task_h_
 #define _ardour_rt_task_h_
 
+#include <functional>
 
 #include "ardour/graphnode.h"