How to compass a rake

I use rake to prepare files for development.
But I can not compile my sass files with a compass

  desc "Compile to compressed css"
  task :compile_compressed do
    #Go to the compass project directory
    Dir.chdir File.join( ENV['base_path'], CONF['dir']['compass'] ) do |dir|
      file_compass_config = "/path/to/some/different/config.rb"

      system "compass compile -c #{file_compass_config} --force"
    end
  end

I am using rvm with gemset.