TypeError: no implicit conversion of nil into String

https://sentry.gitlab.net/gitlab/gitlabcom/issues/2684434/?referrer=gitlab_plugin

TypeError: no implicit conversion of nil into String
  time.rb:372:in `_parse'
    d = Date._parse(date, comp)
  time.rb:372:in `parse'
    d = Date._parse(date, comp)
  gitlab/github_import/representation/pull_request_review.rb:32:in `from_json_hash'
    hash[:submitted_at] = Time.parse(hash[:submitted_at]).in_time_zone
  gitlab/github_import/object_importer.rb:30:in `import'
    object = representation_class.from_json_hash(hash)
  gitlab/github_import/rescheduling_methods.rb:31:in `try_import'
    import(*args)
...
(84 additional frame(s) were not displayed)

no implicit conversion of nil into String