Strange behaviour of assert_redirected_to

I have such lines in routes.rb:

resource :order
match '/' => 'orders#new', :constraints => { :subdomain => /.+/ }

And in controller I have:

redirect_to new_order_path(:subdomain => current_subdomain)

And I write such test:

@request.host = "subdomain." + @request.host
get :test
assert_redirected_to(:action => :new)

So I get an error:

Expected response to be a redirect to <http://subdomain.test.host/> but was a redirect to <http://subdomain.test.host/order/new>

If I delete match '/' from routes it is ok.

I think that new_order_path can recognize that it is / but redirect_to can't.

Rails 3, Ruby 1.9.

subdomain realisation from railscast: http://railscasts.com/episodes/221-subdomains-in-rails-3