diff --git a/xwords4/relay/mlock.h b/xwords4/relay/mlock.h
index 34e5a601c..dd5ba6efc 100644
--- a/xwords4/relay/mlock.h
+++ b/xwords4/relay/mlock.h
@@ -24,6 +24,7 @@
 #include <pthread.h>
 
 #include "xwrelay_priv.h"
+#include "cref.h"
 
 class MutexLock {
  public: