diff --git a/dnsapi/dns_dynv6.sh b/dnsapi/dns_dynv6.sh
index a8bb2adf..b15fe36f 100644
--- a/dnsapi/dns_dynv6.sh
+++ b/dnsapi/dns_dynv6.sh
@@ -16,7 +16,7 @@ dynv6_api="https://dynv6.com/api/v2"
 # Please Read this guide first: https://github.com/Neilpang/acme.sh/wiki/DNS-API-Dev-Guide
 #Usage: dns_dynv6_add  _acme-challenge.www.domain.com  "XKrxpRBosdIKFzxW_CT3KLZNf6q0HG9i01zxXp5CPBs"
 dns_dynv6_add() {
-  fulldomain=$(echo "$1" | tr 'A-Z' 'a-z')
+  fulldomain=$(echo "$1" | tr '[:upper:]' '[:lower:]')
   txtvalue="$2"
   _info "Using dynv6 api"
   _debug fulldomain "$fulldomain"
@@ -50,7 +50,7 @@ dns_dynv6_add() {
 #Usage: fulldomain txtvalue
 #Remove the txt record after validation.
 dns_dynv6_rm() {
-  fulldomain=$(echo "$1" | tr 'A-Z' 'a-z')
+  fulldomain=$(echo "$1" | tr '[:upper:]' '[:lower:]')
   txtvalue="$2"
   _info "Using dynv6 API"
   _debug fulldomain "$fulldomain"