activeadmin: css not getting precompiled

I have a standard Rails 3.1.1.rc1 application set up. The only thing I have done to this point:

• rails new myapp -T -d postgresql
• Edited gemfile to add active_admin and a few other gems
• Moved sass-rails out of assets group as per issue #447
• Set up a very simple index page and set root :to => this page, which has no dynamic content
• migrated the database and run in development mode. Everything runs fine.
• Send the app over to the production server (Apache2, Passenger, Postgresql), migrate database and access the /admin

At this point I get a 500 error. So checking the production log, I get

ActionView::Template::Error (active_admin.css isn't precompiled):
    6:   <title><%= [@page_title, active_admin_application.site_title].compact.join(" | ") %></title>
    7:
    8:   <% ActiveAdmin.application.stylesheets.each do |path| %>
    9:     <%= stylesheet_link_tag path %>
    10:   <% end %>
    11:   <% ActiveAdmin.application.javascripts.each do |path| %>
    12:     <%= javascript_include_tag path %>

So, I open up a terminal and do

rake RAILS_ENV=production assets:precompile

And all the assets are compiled, except for the active_admin.css. I can see that the active_admin.css.scss file is in app/assets/stylesheets but for some reason it is being skipped.

Here is the gemfile:

gem 'rails', '3.1.1.rc1'

gem 'pg'
gem 'sass-rails', "  ~> 3.1.0"
group :assets do
  gem 'therubyracer'
#  gem 'sass-rails', "  ~> 3.1.0"
  gem 'coffee-rails', "~> 3.1.0"
  gem 'uglifier', '>= 1.0.3'
end

gem 'jquery-rails'
gem 'ancestry'
gem 'cancan', :git => "git://github.com/ryanb/cancan.git", :branch => "2.0"
gem 'friendly_id'
gem 'role_model'
gem 'activeadmin'

group :development, :test do
  gem 'guard-rspec'
end

group :development do
  gem 'rspec-rails'
  gem 'unicorn'
  gem 'capistrano'
end

group :test do
  gem 'rspec'
  gem 'webrat'
  gem 'factory_girl_rails'
end

Other than the above gemfile and very basic index page I have made no modifications to the application.