nested destroy problem

bar.rb

class Bar < ActiveRecord::Base
  acts_as_nested_set :dependent => :destroy
  belongs_to :foo
end

console_output.txt

> Foo.destroy_all
ActiveRecord::RecordNotFound: Couldn't find Bar with ID=2

foo.rb

class Foo < ActiveRecord::Base
  has_many :bars, :dependent => :destroy
  has_one :root, :class_name => 'Bar', :conditions => "parent_id is null"
end