rails 3.1 can't compile assets on prod due to asset host config

My production asset_host config looks like this:

  config.action_controller.asset_host = Proc.new { |source, request| 
    if request.ssl? 
      "#{request.protocol}#{request.host_with_port}" 
    else 
      "#{request.protocol}assets#{(source.length % 4) + 1}.example.com" 
    end 
  } 

...which is more or less straight from the docs:

http://api.rubyonrails.org/classes/ActionView/Helpers/AssetTagHelper.html

When I go to assets:precompile, I get this:

$ RAILS_ENV=production bundle exec rake assets:precompile 
rake aborted! 
This asset host cannot be computed without a request in scope. Remove 
the second argument to your asset_host Proc if you do not need the 
request. 

....except that I can't really remove the 2nd arg because I need to know if the request is ssl or not. That said, I understand that a request isn't present during the rake task to generate the assets....

So how do I get out of this catch 22?