diff --git a/libs/ardour/ardour/butler.h b/libs/ardour/ardour/butler.h
index 81159f7d3c..97020bed66 100644
--- a/libs/ardour/ardour/butler.h
+++ b/libs/ardour/ardour/butler.h
@@ -20,6 +20,8 @@
 #ifndef __ardour_butler_h__
 #define __ardour_butler_h__
 
+#include <pthread.h>
+
 #include <glibmm/threads.h>
 
 #ifdef WIN32