Uninitialized constant Admin::Admin

Using Ruby: 2.3.1p112 and Rails: 3.2.12

I'm trying call a demo method in my controller. So, in my _form.html.erb I have:

<%= link_to 'Demo', "/admin/clinics/"+@clinic.id.to_s+"/demo" %>

In my routes.rb:

match "/admin" => "admin#index", :as => :adminnamespace :admin do    resources :admin_users    resources :health_plan_tables    resources :health_aid_tables    resources :clients    resources :clinics    resources :specialties    resources :qualifications    resources :profissionals    resources :addresses    resources :documents    resources :banners    root :to => 'banners#index'    get 'logout' => 'devise/sessions#destroy'    get 'clinics/:id/demo', to: 'admin/clinics#demo', as: 'demo'end

My clinics_controller.rb is inside the folder controllers/admin, and I just have:

def demo    print "hello"end 

So, when I click on link, error message appears Uninitialized constant Admin::Admin.Any ideia how to fix it?