diff --git a/lightningd/subd.c b/lightningd/subd.c
index 778811456b37..79534c373040 100644
--- a/lightningd/subd.c
+++ b/lightningd/subd.c
@@ -944,7 +944,7 @@ char *opt_subd_dev_disconnect(const char *optarg, struct lightningd *ld)
 /* If test specified that this disconnection should cause permanent failure */
 bool dev_disconnect_permanent(struct lightningd *ld)
 {
-	char permfail[strlen("PERMFAIL")];
+	char permfail[sizeof("PERMFAIL") - 1];
 	int r;
 
 	if (ld->dev_disconnect_fd == -1)