Validations of acceptance not working

Hi,
i have model like

class User < ActiveRecord::Base
  attr_accessible :username, :email, :password, :password_confirmation, :remember_me
  attr_accessor :terms

  validates :username,
            :presence => true,
            :format => { :with => /\A[a-z0-9\-]+\z/i },
            :uniqueness => true,
            :length => 3..20

  validates :email,
            :presence => true,
            :format => { :with => /^([\w\.%\+\-]+)@([\w\-]+\.)+([\w]{2,})$/i },
            :uniqueness => true

  validates :password,
            :presence => true,
            :confirmation => true,
            :length => 6..20

  validates :terms,
            :acceptance => true

end

Everything is ok, but validation of :terms does not working.