rails i18n redis ERR unknown command [] railscast 256

Following this RailsCast : http://railscasts.com/episodes/256-i18n-backends but using Rails 5.2, I raise this error :

Redis::CommandError in Pages#home<br>ERR unknown command '[]'

In config/initializers/i18n_backend.rb

TRANSLATION_STORE = Redis.new seems causing this problem.

Whereas TRANSLATION_STORE = {} works like a charm.

But without Redis!

Any hint?