SystemStackError with RSpec mocks any_instance and DataMapper validators

Gemfile

source 'http://rubygems.org'

DM_VERSION = "~> 1.2.0"

gem 'dm-core', DM_VERSION
gem 'dm-validations', DM_VERSION
gem 'dm-sqlite-adapter', DM_VERSION
gem 'dm-migrations', DM_VERSION
gem 'rspec'

Gemfile.lock

GEM
  remote: http://rubygems.org/
  specs:
    addressable (2.2.7)
    data_objects (0.10.8)
      addressable (~> 2.1)
    diff-lcs (1.1.3)
    dm-core (1.2.0)
      addressable (~> 2.2.6)
    dm-do-adapter (1.2.0)
      data_objects (~> 0.10.6)
      dm-core (~> 1.2.0)
    dm-migrations (1.2.0)
      dm-core (~> 1.2.0)
    dm-sqlite-adapter (1.2.0)
      dm-do-adapter (~> 1.2.0)
      do_sqlite3 (~> 0.10.6)
    dm-validations (1.2.0)
      dm-core (~> 1.2.0)
    do_sqlite3 (0.10.8)
      data_objects (= 0.10.8)
    rspec (2.9.0)
      rspec-core (~> 2.9.0)
      rspec-expectations (~> 2.9.0)
      rspec-mocks (~> 2.9.0)
    rspec-core (2.9.0)
    rspec-expectations (2.9.1)
      diff-lcs (~> 1.1.3)
    rspec-mocks (2.9.0)

PLATFORMS
  ruby

DEPENDENCIES
  dm-core (~> 1.2.0)
  dm-migrations (~> 1.2.0)
  dm-sqlite-adapter (~> 1.2.0)
  dm-validations (~> 1.2.0)
  rspec

stack_error_spec.rb

require 'rubygems'
require 'bundler/setup'
require 'dm-core'
require 'dm-validations'
require 'rspec'
require 'dm-migrations'

DataMapper.setup(:default, 'sqlite::memory:')

class Post
  include DataMapper::Resource

  property :id, Serial
  property :title, String

  validates_uniqueness_of :title
end

DataMapper.auto_migrate!


describe Post do
  it "okay with validators being mocked" do
    DataMapper::Validations::UniquenessValidator.any_instance.stub(:valid?).and_return(false)

    p = Post.new
    p.title = "foobar"
    p.should_not be_valid

  end

  it "should still work after mocks have been removed" do
    p = Post.new
    p.title = "dog"
    p.valid?.should be_true
  end
end