File locking fails

Zendesk ticket (internal link): https://gitlab.zendesk.com/agent/tickets/93159

When trying to lock a directory in GitLab EE Premium 10.6.0, it fails and we get the following error in the logs:

==> /var/log/gitlab/gitlab-rails/production.log <==

NoMethodError (undefined method `id' for nil:NilClass):
  lib/gitlab/i18n.rb:50:in `with_locale'
  lib/gitlab/i18n.rb:56:in `with_user_locale'
  app/controllers/application_controller.rb:338:in `set_locale'
  lib/gitlab/middleware/multipart.rb:95:in `call'
  lib/gitlab/request_profiler/middleware.rb:14:in `call'
  lib/gitlab/middleware/go.rb:17:in `call'
  lib/gitlab/etag_caching/middleware.rb:11:in `call'
  lib/gitlab/middleware/read_only/controller.rb:28:in `call'
  lib/gitlab/middleware/read_only.rb:16:in `call'
  lib/gitlab/request_context.rb:18:in `call'
  lib/gitlab/metrics/requests_rack_middleware.rb:27:in `call'
  lib/gitlab/middleware/release_env.rb:10:in `call'

File locking seems to still work with individual files, and only fails with directories. I've reproduced the problem in 10.5.6-ee Premium as well.