Singular Resource mapping to plural controller in rails 3

resources :users do

  resource :wall do
    post :add_comment
  end
end

end

and the route generated is

user_wall POST /users/:user_id/wall(.:format) {:controller=>"walls", :action=>"create"}

new_user_wall GET /users/:user_id/wall/new(.:format) {:controller=>"walls", :action=>"new"}

edit_user_wall GET /users/:user_id/wall/edit(.:format) {:controller=>"walls", :action=>"edit"}

user_wall GET /users/:user_id/wall(.:format) {:controller=>"walls", :action=>"show"}

user_wall PUT /users/:user_id/wall(.:format) {:controller=>"walls", :action=>"update"}

user_wall DELETE /users/:user_id/wall(.:format) {:controller=>"walls", :action=>"destroy"}

Why the Singular Resource maps to plural controller "walls" not the singular form "wall"
Is this a bug ?