routes

namespace :users do
  resources :clients do
    scope 'applications', as: 'applications', klass: Application do
      resources :submit_applications, only: [:update]
    end
    resources :applications
  end
end

= form_for application, url: users_client_application_submit_application_path(client, application) do |f|
  = f.submit


=> undefined method 'integer' for <Class<Class:4245234524t>2345234523452>

Yeah, I think this was fixed later, but I'm not sure. Looks like you should pass a string and call constantize.