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.