async_sinatra cannot determine application with Sinatra 1.3.3

app.rb

require 'sinatra/base'
require 'sinatra/async'

class MyApp < Sinatra::Base
  register Sinatra::Async

  configure do
    set :raise_errors, false
    set :show_exceptions, false
    enable :logging  
  end

  aget '/' do
    body 'Big time!'
  end
end

app_spec.rb

require 'rspec'
require 'test/unit'
require 'eventmachine'
require 'sinatra/async/test'
require_relative '../app'

ENV['RACK_ENV'] = 'test'

RSpec.configure do |config|
  config.include Test::Unit::Assertions
  config.include Sinatra::Async::Test::Methods  
end

describe 'MyApp', :type => :request do
  def app
    MyApp.new
  end

  it 'can access the base route.' do
    aget '/'
    last_response.status.should eq 200
  end
end

output

Failures:

  1) MyApp can access the base route.
     Failure/Error: aget '/'
     RuntimeError:
       Cannot determine sinatra application from #<struct Sinatra::ExtendedRack app=#<Rack::Head:0xa0c5490 @app=#<Sinatra::CommonLogger:0xa0c54a4 @app=#<Rack::Logger:0xa0c54e0 @app=#<Rack::Protection::FrameOptions:0xa0c5580 @app=#<Rack::Protection::IPSpoofing:0xa0c55f8 @app=#<Rack::Protection::JsonCsrf:0xa0c574c @app=#<Rack::Protection::PathTraversal:0xa0c57d8 @app=#<Rack::Protection::XSSHeader:0xa0c5878 @app=#<MyApp:0xa0c61c4 @default_layout=:layout, @app=nil, @template_cache=#<Tilt::Cache:0xa0c61b0 @cache={}>>, @options={:reaction=>:drop_session, :logging=>true, :message=>"Forbidden", :encryptor=>Digest::SHA1, :session_key=>"rack.session", :status=>403, :allow_empty_referrer=>true, :xss_mode=>:block, :except=>[:session_hijacking, :remote_token]}>, @options={:reaction=>:drop_session, :logging=>true, :message=>"Forbidden", :encryptor=>Digest::SHA1, :session_key=>"rack.session", :status=>403, :allow_empty_referrer=>true, :except=>[:session_hijacking, :remote_token]}>, @options={:reaction=>:drop_session, :logging=>true, :message=>"Forbidden", :encryptor=>Digest::SHA1, :session_key=>"rack.session", :status=>403, :allow_empty_referrer=>true, :except=>[:session_hijacking, :remote_token]}>, @options={:reaction=>:drop_session, :logging=>true, :message=>"Forbidden", :encryptor=>Digest::SHA1, :session_key=>"rack.session", :status=>403, :allow_empty_referrer=>true, :except=>[:session_hijacking, :remote_token]}>, @options={:reaction=>:drop_session, :logging=>true, :message=>"Forbidden", :encryptor=>Digest::SHA1, :session_key=>"rack.session", :status=>403, :allow_empty_referrer=>true, :xss_mode=>:block, :frame_options=>:sameorigin, :except=>[:session_hijacking, :remote_token]}>, @level=1>, @logger=nil>>>
     # ./app_spec.rb:20:in `block (2 levels) in <top (required)>'

Finished in 0.05393 seconds
1 example, 1 failure

Same here. Scratching my head over this :(

FYI if others encounter this:

There's a PR here that's never been merged.

Just get the fix from Soffes' fork.