diff --git a/.travis.yml b/.travis.yml
index 0667919d..fd4ee25c 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -41,7 +41,7 @@ script:
   - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then ~/shfmt -l -w -i 2 . ; fi
   - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then git diff --exit-code && echo "shfmt OK" ; fi
   - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then shellcheck -V ; fi
-  - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then shellcheck -e SC2021,SC2126,SC2034 **/*.sh && echo "shellcheck OK" ; fi
+  - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then shellcheck -e SC2126,SC2034 **/*.sh && echo "shellcheck OK" ; fi
   - cd ..
   - git clone https://github.com/Neilpang/acmetest.git && cp -r acme.sh acmetest/ && cd acmetest
   - if [[ "$TRAVIS_OS_NAME" == "linux" ]] && [[ "$NGROK_TOKEN" ]]; then sudo NGROK_TOKEN="$NGROK_TOKEN" ./letest.sh ; fi
diff --git a/acme.sh b/acme.sh
index 06b14ef6..d0f97b1a 100755
--- a/acme.sh
+++ b/acme.sh
@@ -573,7 +573,7 @@ _createkey() {
 _is_idn() {
   _is_idn_d="$1"
   _debug2 _is_idn_d "$_is_idn_d"
-  _idn_temp=$(printf "%s" "$_is_idn_d" | tr -d '[0-9]' | tr -d '[a-z]' | tr -d '[A-Z]' | tr -d '.,-')
+  _idn_temp=$(printf "%s" "$_is_idn_d" | tr -d '0-9' | tr -d 'a-z' | tr -d 'A-Z' | tr -d '.,-')
   _debug2 _idn_temp "$_idn_temp"
   [ "$_idn_temp" ]
 }
diff --git a/dnsapi/dns_lexicon.sh b/dnsapi/dns_lexicon.sh
index 4ab65645..0398bd2e 100755
--- a/dnsapi/dns_lexicon.sh
+++ b/dnsapi/dns_lexicon.sh
@@ -30,7 +30,9 @@ dns_lexicon_add() {
   _savedomainconf PROVIDER "$PROVIDER"
   export PROVIDER
 
-  Lx_name=$(echo LEXICON_"${PROVIDER}"_USERNAME | tr '[a-z]' '[A-Z]')
+  # e.g. busybox-ash does not know [:upper:]
+  # shellcheck disable=SC2018,SC2019
+  Lx_name=$(echo LEXICON_"${PROVIDER}"_USERNAME | tr 'a-z' 'A-Z')
   Lx_name_v=$(eval echo \$"$Lx_name")
   _debug "$Lx_name" "$Lx_name_v"
   if [ "$Lx_name_v" ]; then
@@ -38,7 +40,8 @@ dns_lexicon_add() {
     eval export "$Lx_name"
   fi
 
-  Lx_token=$(echo LEXICON_"${PROVIDER}"_TOKEN | tr '[a-z]' '[A-Z]')
+  # shellcheck disable=SC2018,SC2019
+  Lx_token=$(echo LEXICON_"${PROVIDER}"_TOKEN | tr 'a-z' 'A-Z')
   Lx_token_v=$(eval echo \$"$Lx_token")
   _debug "$Lx_token" "$Lx_token_v"
   if [ "$Lx_token_v" ]; then
@@ -46,7 +49,8 @@ dns_lexicon_add() {
     eval export "$Lx_token"
   fi
 
-  Lx_password=$(echo LEXICON_"${PROVIDER}"_PASSWORD | tr '[a-z]' '[A-Z]')
+  # shellcheck disable=SC2018,SC2019
+  Lx_password=$(echo LEXICON_"${PROVIDER}"_PASSWORD | tr 'a-z' 'A-Z')
   Lx_password_v=$(eval echo \$"$Lx_password")
   _debug "$Lx_password" "$Lx_password_v"
   if [ "$Lx_password_v" ]; then
@@ -54,7 +58,8 @@ dns_lexicon_add() {
     eval export "$Lx_password"
   fi
 
-  Lx_domaintoken=$(echo LEXICON_"${PROVIDER}"_DOMAINTOKEN | tr '[a-z]' '[A-Z]')
+  # shellcheck disable=SC2018,SC2019
+  Lx_domaintoken=$(echo LEXICON_"${PROVIDER}"_DOMAINTOKEN | tr 'a-z' 'A-Z')
   Lx_domaintoken_v=$(eval echo \$"$Lx_domaintoken")
   _debug "$Lx_domaintoken" "$Lx_domaintoken_v"
   if [ "$Lx_domaintoken_v" ]; then