diff --git a/sys/kern/kern_jail.c b/sys/kern/kern_jail.c
--- a/sys/kern/kern_jail.c
+++ b/sys/kern/kern_jail.c
@@ -991,7 +991,7 @@
 {
 	char id_str[7];
 
-	ksnprintf(id_str, 6, "%d", pr->pr_id);
+	ksnprintf(id_str, sizeof(id_str), "%d", pr->pr_id);
 
 	pr->pr_sysctl_ctx = (struct sysctl_ctx_list *) kmalloc(
 		sizeof(struct sysctl_ctx_list), M_PRISON, M_WAITOK | M_ZERO);