Why is my Rails custom error page ignoring a custom layout?

I have custom dynamic error pages in my Rails 3.2 app.

To ensure that the these error pages don't themselves throw errors, they use a simplified application layout that removes, among other things, dynamic user-account information (ie. like a user avatar in the header) using a simplified layout application-limited.html.erb.

 # routes.rb
 get '/404', to: 'errors#page_not_found', as: :page_not_found

 # errors_controller.rb
 class ErrorsController < ApplicationController
   def page_not_found
     render status: 404, layout: 'application-limited'
   end
 end

When I test by visiting /404 or /500 the error page renders correctly with the limited layout. However when I test by visiting a non-existent page /foobar I get the correct dynamic 404 error page, but strangely the normal application layout is rendered, rather than the simplified application-limited layout.

What am I missing here?