Problem: not_found doesn't work at all

config.ru

# coding: utf-8
require 'rack/urlmap'
require File.join(File.dirname(__FILE__), 'app')
require File.join(File.dirname(__FILE__), 'api', 'v1')

routes = Rack::URLMap.new(
  '/'        => Sinatra::Application,
  '/api/v1/' => ApiV1
)

set :environment, :production
run routes

v1.rb

require File.join(File.dirname(__FILE__), '..', 'config', 'environment')
require 'sinatra/base'

class ApiV1 < Sinatra::Base
  before do
    content_type :json
  end

  # snip

  not_found do
    Jbuilder.encode do |json|
      json.error 'not found'
    end
  end
end