Getting an exception with Refile Gem when user avatar image isn't available

module MessagesHelper

  def recipients_options
    s = ''
    users = User.all + BizUser.all; users.each do |user|
      s << "<option value='#{user.id}'
data-img-src='#{attachment_url(@user, user.email, :profile_avatar,
:fill, 50, 50)}'>#{user.username}</option>"
    end
    s.html_safe
  end
end

How would I pass if nil to the attachment_url for user :profile_avatar?
when the image isn't available.