One test affects another

I have the following class:

class Brewer < ActiveRecord::Base
  class EmailNotFound < StandardError; end
  class << self
    def find_by_email(*args)
      b = super(*args)
      raise EmailNotFound if b.nil?
      return b
    end
  end
end

And the following two tests:

require File.dirname(__FILE__) + '/../test_helper'

class BrewerTest < Test::Unit::TestCase
  fixtures :brewers

  def test_should_find_brewer_with_valid_email
    assert_equal brewers(:active_brewer),
Brewer.find_by_email(brewers(:active_brewer).email)
  end

  def test_should_raise_email_not_found_exception_with_invalid_email
    assert_raises Brewer::EmailNotFound do
      Brewer.find_by_email('NotAnEmail')
    end
  end
end

The tests fail like this:

Loaded suite test/unit/brewer_test
Started
.F
Finished in 0.428367 seconds.

  1) Failure:
test_should_raise_email_not_found_exception_with_invalid_email(BrewerTest)
[test/unit/brewer_test.rb:207]:
<Brewer::EmailNotFound> exception expected but none was thrown.

2 tests, 2 assertions, 1 failures, 0 errors

If I comment out test_should_find_brewer_with_valid_email, the other
test passes.

What's going on here?

Regards,
--Dean