docx_converter | Ruby gem converting Word docx files into html or LaTeX | Document Editor library

TL;DR:

I should have used the docx gem and not the (older) docx_converter. This seems much more compatible with Rails 5.2.

I'm learning rails and have a working application. However, when I add the gem docx_converter to my Gemfile (so I can convert Word files to markdown to work on further), the server fails to launch.

The application has three models: Company, which has_many Users, which has_many Projects. (Users is controlled by Devise, and Company has_many Projects through Users.) When a user creates a project, they can upload a file to that project using active_storage, and it's stored on disk in my dev environment (which is what I'm concerned with here).

I have no problem with any of the above - I can create users and projects, and add and remove files from the projects.

However, if I add gem 'docx_converter' to my Gemfile and run bundle update, I can no longer run the rails server. Immediately before this action, all is fine. Immediately after, the server won't run and the traceback (last few lines) is: