diff --git a/dnsapi/dns_nsupdate.sh b/dnsapi/dns_nsupdate.sh
index cc57cc38..8d7fe2c0 100755
--- a/dnsapi/dns_nsupdate.sh
+++ b/dnsapi/dns_nsupdate.sh
@@ -6,7 +6,7 @@ Docs: github.com/acmesh-official/acme.sh/wiki/dnsapi#dns_nsupdate
 Options:
  NSUPDATE_SERVER Server hostname. Default: "localhost".
  NSUPDATE_SERVER_PORT Server port. Default: "53".
- NSUPDATE_KEY File path to TSIG key. Default: ""
+ NSUPDATE_KEY File path to TSIG key. Default: "". Optional.
  NSUPDATE_ZONE Domain zone to update. Optional.
 '