diff --git a/libs/midi++2/midi++/port.h b/libs/midi++2/midi++/port.h
index 599fabaa87..439760115c 100644
--- a/libs/midi++2/midi++/port.h
+++ b/libs/midi++2/midi++/port.h
@@ -24,6 +24,8 @@
 
 #include <jack/types.h>
 
+#include <pthread.h>
+
 #include "pbd/xml++.h"
 #include "pbd/crossthread.h"
 #include "pbd/signals.h"