提供无位置。无法为 Rails 5.2 构建 URI

问：

我对 Ruby 编程很陌生。将应用程序从 Rails 4 升级到 5.2。运行 RSpec 时出现以下错误。Rails 版本和其他更改已经完成。不确定如何解决错误。

Failure/Error: = image_tag current_user.gravatar_url, class: 'gravatar'

ActionView::Template::Error: Nil location provided. Can't build URI.
      # ./app/views/layouts/_navigation.html.haml:25:in `_app_views_layouts__navigation_html_haml___4566101014686141882_47249923845420'
      # ./app/views/layouts/application.html.haml:15:in `_app_views_layouts_application_html_haml__778403214308649385_47249873224260'
      # ./spec/controllers/clones_controller_spec.rb:7:in `block (3 levels) in <top (required)>'
      # ------------------
      # --- Caused by: ---
      # ArgumentError:
      #   Nil location provided. Can't build URI.
      #   ./app/views/layouts/_navigation.html.haml:25:in `_app_views_layouts__navigation_html_haml__

Gravatar 定义：user.rb

def gravatar_url
  md5 = Digest::MD5.hexdigest(email)
  "https://www.gravatar.com/avatar/#{md5}"
end

_navigation.html.haml文件：

%ul.nav.navbar-nav.pull-right{:style => 'font-size:12px'}
        - if user_signed_in?
          %li.dropdown.profile
            %a.dropdown-toggle{:'data-toggle' => 'dropdown'}
              = image_tag current_user.gravatar_url, class: 'gravatar'
              = current_user.name
              %b.caret
            %ul.dropdown-menu
              %li= link_to 'Permissions', command_permissions_path if can? :index, CommandPermission
              %li= link_to 'Logout', logout_path, :method => :delete