(SystemStackError) Unit Tests Not working

Hi,

Please Help, I am trying to run tests on my website but it fails each
and every time I think its not even running test.
ruby version: ruby 1.8.6 (2008-03-03 patchlevel 114) [x86_64-linux]
rails version: Rails 2.3.5

when I did this from console
ruby test/units/activity_test.rb

it throws this error (and no other test is running)
/usr/lib64/ruby/gems/1.8/gems/mysqlplus-0.1.1/lib/mysqlplus.rb:18:
warning: parenthesize argument(s) for future version
/usr/lib/ruby/site_ruby/1.8/rubygems/source_index.rb:324:in `search':
stack level too deep (SystemStackError)
        from /usr/lib/ruby/site_ruby/1.8/rubygems/source_index.rb:
323:in `select'
        from /usr/lib/ruby/site_ruby/1.8/rubygems/source_index.rb:
323:in `search'
        from /usr/lib64/ruby/gems/1.8/gems/rails-2.3.2/lib/rails/
vendor_gem_source_index.rb:119:in `search'
        from /usr/lib64/ruby/gems/1.8/gems/rails-2.3.2/lib/rails/
gem_dependency.rb:78:in `specification'
        from /usr/lib64/ruby/gems/1.8/gems/rails-2.3.2/lib/rails/
plugin/locator.rb:81:in `plugins'
        from /usr/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:
31:in `inject'
        from /usr/lib64/ruby/gems/1.8/gems/rails-2.3.2/lib/rails/
plugin/locator.rb:81:in `each'
        from /usr/lib64/ruby/gems/1.8/gems/rails-2.3.2/lib/rails/
plugin/locator.rb:81:in `inject'
         ... 2476 levels...
        from ./test/unit/../test_helper.rb:2:in `require'
        from ./test/unit/../test_helper.rb:2
        from test/unit/activity_test.rb:1:in `require'
        from test/unit/activity_test.rb:1

My test case is very simple

this is my activity_test.rb

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

class ActivityTest < ActiveSupport::TestCase

  def setup
  end

  def test_per_page
    assert_instance_of Fixnum, Activity.per_page
  end
end

I do not having any idea why its not running. Please help

Thanks
Sumeet