form_for doesn't work with new singleton nested resource

Using a singleton nested resource, such as:

map.resources :users do |users|
  users.resource :position
end

form_for doesn't work when the nested resource is being added.

For example, in the controller:

def new
  @user = User.find(params[:user_id])
  @position = @user.build_position
end

and in the view:

form_for [:user, @member] ...

will throw an exception: undefined method user_positions_path'

The edit action is identical except the nested resource exists, so in the controller: @position = @user.position instead of @user.build_position. Using form_for [:user, @member] in the edit view creates a form with the correct singular route and put method. It fails to work only when the nested resource is a new record.

I believe the problem is in the polymorphic_url helper, used by apply_form_for_options! in form_for. The helper determines inflection based on new_record? for some reason. If the record is new the inflection is plural. This obviously fails for new singleton nested resources. Here is the pertinent section of polymorphic_url:

inflection =
  case
  when options[:action].to_s == "new"
    args.pop
    :singular
  when record.respond_to?(:new_record?) && record.new_record?
    args.pop
    :plural
  else
    :singular
  end

It's possible to work around this using a different form_for syntax to explicitly set the URL, but this seems like a bug because the behavior is inconsistent between the new and edit actions.