ArgumentError: string contains null byte

https://sentry.gitlab.net/gitlab/gitlabcom/issues/565802/

ArgumentError: string contains null byte
  gitlab/git_ref_validator.rb:16:in `valid_name?'
    Rugged::Reference.valid_name? "refs/heads/#{ref_name}"
  gitlab/git_ref_validator.rb:16:in `validate'
    Rugged::Reference.valid_name? "refs/heads/#{ref_name}"
  projects/compare_controller.rb:71:in `valid_ref?'
    Gitlab::GitRefValidator.validate(ref_name)
  projects/compare_controller.rb:75:in `block in validate_refs!'
    valid = [head_ref, start_ref].map { |ref| valid_ref?(ref) }
  projects/compare_controller.rb:75:in `map'
    valid = [head_ref, start_ref].map { |ref| valid_ref?(ref) }
...
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ArgumentError: string contains null byte