diff --git a/libs/ardour/ardour/port_engine_shared.h b/libs/ardour/ardour/port_engine_shared.h
index 6dbd012961..3ca06d4d06 100644
--- a/libs/ardour/ardour/port_engine_shared.h
+++ b/libs/ardour/ardour/port_engine_shared.h
@@ -34,6 +34,8 @@
 #include "ardour/port_engine.h"
 #include "ardour/types.h"
 
+#include "pbd/g_atomic_compat.h"
+
 namespace ARDOUR {
 
 class PortEngineSharedImpl;