NoMethodError: undefined method `downcase' for nil:NilClass

https://sentry.gitlab.net/gitlab/gitlabcom/issues/4034661/?referrer=gitlab_plugin

NoMethodError: undefined method `downcase' for nil:NilClass
  app/models/user.rb:497:in `map'
    scope :by_emails, -> (emails) { joins(:emails).where(emails: { email: Array(emails).map(&:downcase) }) }
  app/models/user.rb:497:in `block in <class:User>'
    scope :by_emails, -> (emails) { joins(:emails).where(emails: { email: Array(emails).map(&:downcase) }) }
  app/models/user.rb:669:in `by_any_email'
    from_emails = by_emails(emails).merge(Email.confirmed)
  app/models/commit_collection.rb:30:in `committers'
    User.by_any_email(emails)
  app/models/merge_request.rb:623:in `committers'
    @committers ||= commits.committers
...
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