rake test problem

Hi

I'm getting weird lock_mutex undefined errors when running tests.

I tried creating a new application having just acts_as_authenticated
installed.

Running rake test returns errors like:

test_should_require_password_on_signup(AccountControllerTest):
NoMethodError: undefined method `lock_mutex' for
ActiveRecord::Base:Class

/usr/lib/ruby/gems/1.8/gems/activerecord-1.15.3/lib/active_record/
base.rb:1235:in `method_missing'
    /usr/lib/ruby/1.8/active_record/fixtures.rb:490:in
`setup_with_fixtures'
    /usr/lib/ruby/1.8/active_record/fixtures.rb:522:in `setup'

I am running rails 1.2.3 with activerecord 1.15.3.