diff --git a/sys/kern/tty_pts.c b/sys/kern/tty_pts.c
index b8edca0d91a..dbbf35cf1a1 100644
--- a/sys/kern/tty_pts.c
+++ b/sys/kern/tty_pts.c
@@ -50,6 +50,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/kernel.h>
 #include <sys/limits.h>
 #include <sys/malloc.h>
+#include <sys/mutex.h>
 #include <sys/poll.h>
 #include <sys/proc.h>
 #include <sys/racct.h>