After upgrading to Rails 6.0.2.1, creating a GoCardless redirect flow failed with GoCardlessPro::InvalidApiUsageError: One of your parameters was incorrectly typed. Rack changed session.id to return a Rack::Session::SessionId object instead of a String; the SDK serialized it as a JSON object and the API rejected it. A maintainer confirmed the diagnosis but declined to auto-cast parameters in the client library.