diff --git a/libs/backends/portaudio/winmmemidi_io.cc b/libs/backends/portaudio/winmmemidi_io.cc
index d845e60551..00e3eb0182 100644
--- a/libs/backends/portaudio/winmmemidi_io.cc
+++ b/libs/backends/portaudio/winmmemidi_io.cc
@@ -20,6 +20,7 @@
 #include <mmsystem.h>
 
 #include <sstream>
+#include <set>
 
 #include "pbd/error.h"
 #include "pbd/compose.h"