Rails - May 2008 - assert_no_difference question

I have the following test code:

assert_no_difference Avatar, :count do
  a = create_avatar(:description => nil)
  assert a.errors.on(:description)
end

further down, create_avatar looks like:

  def create_avatar(options = {})
	Avatar.create(:description => "Foo Descr.").merge(options)
  end

(Borrowed from the user test in acts_as_auth...)

It seems to pass, as my avatar model has:

validates_presence_of :description written.

However, if say I try to force it to fail with assert_difference... it
still passes.

What did I miss?
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