IOError: not opened for reading

https://sentry.gitlab.net/gitlab/gitlabcom/issues/515862/

IOError: not opened for reading
  active_support/core_ext/object/json.rb:128:in `each'
    to_a.as_json(options)
  active_support/core_ext/object/json.rb:128:in `to_a'
    to_a.as_json(options)
  active_support/core_ext/object/json.rb:128:in `as_json'
    to_a.as_json(options)
  active_support/core_ext/object/json.rb:159:in `block in as_json'
    Hash[subset.map { |k, v| [k.to_s, options ? v.as_json(options.dup) : v.as_json] }]
  active_support/core_ext/object/json.rb:159:in `each'
    Hash[subset.map { |k, v| [k.to_s, options ? v.as_json(options.dup) : v.as_json] }]
...
(195 additional frame(s) were not displayed)

IOError: not opened for reading

/cc @DouweM