diff --git a/kern/tty_subr.c b/kern/tty_subr.c
--- a/kern/tty_subr.c
+++ b/kern/tty_subr.c
@@ -58,6 +58,8 @@
 		clist_free_cblocks(cl);
 		return;
 	}
+	if (ccmax < 0)
+		ccmax = 0;
 	data = kmalloc(ccmax * sizeof(*data), M_TTYS, M_INTWAIT|M_ZERO);
 	/* NOTE: cl fields may now be different due to blocking */