diff --git a/sys/sys/tty.h b/sys/sys/tty.h
index 81f4f9c6b94..ad74f90bfe1 100644
--- a/sys/sys/tty.h
+++ b/sys/sys/tty.h
@@ -69,6 +69,7 @@ struct tty;
 struct pps_state;
 struct cdev;
 struct cdevsw;
+struct thread;
 
 typedef int t_open_t(struct tty *, struct cdev *);
 typedef void t_close_t(struct tty *);