Running Sinatra app inside Thread doesn't work

Finally I run the other ruby process in a Thread but from Sinatra application and it works just fine.

class App < Sinatra::Base
    threads = []

    threads <<
      Thread.new do
        Some::Other::Thing
      rescue StandardError => e
        $stderr << e.message
        $stderr << e.backtrace.join("\n")
      end

    trap('INT') do
      puts 'trapping'
      threads.each do |t|
        puts 'killing'
        Thread.kill t
      end
    end

    run!
end

And I added a control when Sinatra app is exited also kill the open thread.