Plexus causes a SystemStackError on Rails 3.2.6

[wingrunr21]

When Plexus is required in a Rails 3.2.6 application, ActiveRecord finds produce the following output:

SystemStackError: stack level too deep
from /Users/wingrunr21/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/pry-0.9.9.6/lib/pry/pry_instance.rb:249

To reproduce:

rails new testapp
cd testapp
echo "gem 'plexus'" >> Gemfile
bundle install
rails g scaffold User first_name:string last_name:string
rake db:migrate
rails console

User.all

[wingrunr21]

The issue is specifically in ext.rb#L38

is_a? is being thrown into an infinite loop. klass in every instance is an ActiveRecord::Relation.