How do I use expect{}.to change().from().to() ?

foo.rb

require 'rspec'

class Foo
  attr_reader :ary
  def initialize
    @ary = []
    @num = 0
  end
  def update
    @ary << @num
    @num += 1
  end
end

describe Foo do
  describe "#update" do
    context 'when first update' do
      subject{ Foo.new }
      it do
        expect{ subject.update }.to change(subject, :ary).from([]).to([0])
      end
    end
  end
end

# result this
# /Users/niku/junk/2011/03% rspec --version
# 2.5.1
# /Users/niku/junk/2011/03% rspec --color --format documentation foo.rb
#
# Foo
#   #update
#     when first update
#       should change #ary (FAILED - 1)
#
# Failures:
#
#   1) Foo#update when first update
#      Failure/Error: expect{ subject.update }.to change(subject, :ary).from([]).to([0])
#        ary should have initially been [], but was [0]
#      # ./foo.rb:20:in `block (4 levels) in <top (required)>'
#
# Finished in 0.00066 seconds
# 1 example, 1 failure