Problem with scope

I have two related models such as this:

class PartCategory < ActiveRecord::Base
  has_many :part_types
  scope :engine, where(:name => 'Engine')
end

class PartType < ActiveRecord::Base
  belongs_to :part_category
end

I would like to add a scope to the PartType model such as:

   scope :engine_parts, lambda { joins(:part_category).engine }

But when I try that, I get the following error:

NoMethodError: undefined method `default_scoped?' for
ActiveRecord::Base:Class
  from /Users/bappelt/RubymineProjects/MyStreetMachine/idunno42!/ruby/
1.9.1/gems/activerecord-3.1.0/lib/active_record/base.rb:1082:in
`method_missing'
  from /Users/bappelt/RubymineProjects/MyStreetMachine/idunno42!/ruby/
1.9.1/gems/activerecord-3.1.0/lib/active_record/relation/
spawn_methods.rb:11:in `merge'
  from /Users/bappelt/RubymineProjects/MyStreetMachine/idunno42!/ruby/
1.9.1/gems/activerecord-3.1.0/lib/active_record/named_scope.rb:182:in
`block in scope'
  from (irb):1
  from /Users/bappelt/RubymineProjects/MyStreetMachine/idunno42!/ruby/
1.9.1/gems/railties-3.1.0/lib/rails/commands/console.rb:45:in `start'
  from /Users/bappelt/RubymineProjects/MyStreetMachine/idunno42!/ruby/
1.9.1/gems/railties-3.1.0/lib/rails/commands/console.rb:8:in `start'
  from /Users/bappelt/RubymineProjects/MyStreetMachine/idunno42!/ruby/
1.9.1/gems/railties-3.1.0/lib/rails/commands.rb:40:in `<top
(required)>'
  from script/rails:6:in `require'
  from script/rails:6:in `<main>'

I don't have a lot of experience with the scope thing, so I am
probably missing something fundamental here. Can someone please tell
me what it is.