diff --git a/scripts/lib/certbot-maybe-renew b/scripts/lib/certbot-maybe-renew
index a69b2ed352..28918a4e0f 100755
--- a/scripts/lib/certbot-maybe-renew
+++ b/scripts/lib/certbot-maybe-renew
@@ -5,7 +5,7 @@ zulip_conf_get_boolean() {
     # `configparser` library's conventions for what counts as true.
     # Treat absent and invalid values as false.
     value=$(crudini --get /etc/zulip/zulip.conf "$1" "$2" 2>/dev/null)
-    case "$(echo "$value" | tr A-Z a-z)" in
+    case "$(echo "$value" | tr '[:upper:]' '[:lower:]')" in
         1|yes|true|on) return 0 ;;
         *) return 1 ;;
     esac