Middleman: Fully automated alternate language links

helper.rb

# Returns a hash of localized paths for a given page
def localized_paths_for(page)
  localized_paths = {}
  (langs).each do |locale|

    # Loop over all pages to find the ones using the same templates (proxied_to) for each language
    sitemap.resources.select do |resource|
      if resource.proxied_to == page.proxied_to && resource.metadata[:options][:lang] == locale
        localized_paths[locale] = resource.url
        break
      end
    end
  end

  localized_paths
end

layout.erb

<% localized_paths_for(current_page).each do |text, path| %>
  <link rel="alternate" hreflang="<%= text %>" href="https://example.com<%= path %>"/>
<% end %>

@xeraa when I try to run my Middleman localhost site with it I am getting:

NoMethodError at /
undefined method `proxied_to' for #<Middleman::Sitemap::Resource path=tos.html>

Do I need to specify something in frontmatter?