ruby-on-rails - Rails 4 - form with namespace model name spliting route name

Question

I have rails 4 app with an admin namespace and a model called Gametime

namespace :admin do
  resources :gametimes
end

the model is game_time.rb

class GameTime < ActiveRecord::Base
end

The index and show action work fine. I am having a problem with the new action.

gametimes_controller.rb

def new
  @gametime = GameTime.new
end

new.html.erb

<% form_for [:admin, @gametime ] do |f| %>
<% end %>

I get this error

 NoMethodError at /admin/gametimes/new 
 undefined method `admin_game_times_path' for #<#<Class:0xae0d74c>:0xae0c2c0

 Request parameters 
 {"action"=>"new", "controller"=>"admin/gametimes"}

The routes show this

  new_admin_gametime GET    /admin/gametimes/new(.:format)       admin/gametimes#new

I don't see why it putting the path as "game_time" rather than "gametime" ?