Error with rails 3 nested routes

The bug. I'm getting this error incorrectly:

No route matches {:action=>"destroy", :controller=>"admin/pledges"}

NOTE: This is triggered by a simple form_for:

<%= form_for @pledge, :url => admin_pledge_path do |f| %>

When I'm almost certain I've set everything up correctly, and to verify, when I do "rake routes" it shows me that this route indeed exists:

              GET    /admin/pledges(.:format)          {:action=>"index", :controller=>"admin/pledges"}
admin_pledges POST   /admin/pledges(.:format)          {:action=>"create", :controller=>"admin/pledges"}

new_admin_pledge GET /admin/pledges/new(.:format) {:action=>"new", :controller=>"admin/pledges"}

              GET    /admin/pledges/:id(.:format)      {:action=>"show", :controller=>"admin/pledges"}
              PUT    /admin/pledges/:id(.:format)      {:action=>"update", :controller=>"admin/pledges"}
 admin_pledge DELETE /admin/pledges/:id(.:format)      {:action=>"destroy", :controller=>"admin/pledges"}

edit_admin_pledge GET /admin/pledges/:id/edit(.:format) {:action=>"edit", :controller=>"admin/pledges"}

Here is the relevant part from my routes file:

namespace :admin do

resources :pledges

end