diff --git a/kern/tty_subr.c b/kern/tty_subr.c
--- a/kern/tty_subr.c
+++ b/kern/tty_subr.c
@@ -296,6 +296,8 @@
 {
 	int c;
 
+	if (cls == cld)
+		return;
 	while ((c = clist_getc(cls)) != -1)
 		clist_putc(c, cld);
 }