--- a/sys/kern/tty_conf.c
+++ b/sys/kern/tty_conf.c
@@ -103,6 +103,11 @@
 			}
 	}
 	else if (discipline >= 0 && discipline < MAXLDISC) {
+		if (bcmp(linesw + discipline, &nodisc, sizeof(nodisc)) != 0) {
+			/* slot occupied: refuse instead of aliasing */
+			lwkt_reltoken(&tty_token);
+			return (-1);
+		}
 		slot = discipline;
 	}