Undefined method 'config' for ActiveModel::Serializer

I am trying to follow several tutorials to connect Rails 5 in API mode with Ember application but in all of them once I create new initializer and use:

ActiveModel::Serializer.config.adapter = :json

I get "undefined method 'config' for ActiveModel::Serializer ..." I am using 'active_model_serializer' gem since Ember expects a json with a root object instead of plain json