create and find_or_create_by behaves differently on model with no :id column

$ rails -v
Rails 2.3.5
$ rails tour
$ cd tour
$ script/generate model country code:string name:string --skip-timestamps

    class CreateCountries < ActiveRecord::Migration
      def self.up
        create_table :countries, :id => false do |t|
          t.string :code
          t.string :name
        end
      end

      def self.down
        drop_table :countries
      end
    end

$ rake db:migrate   
$ script/console
Loading development environment (Rails 2.3.5)
>> Country.create(:code => 'ZW', :name => 'Zimbabwe')
=> #<Country code: "ZW", name: "Zimbabwe">

define a primary key in country.rb:

class Country < ActiveRecord::Base
  set_primary_key :code
end

back to console:

>> reload!
Reloading...
=> true
>> Country.find_or_create_by_code('X1')
=> #<Country code: "X1", name: nil>
>> Country.create(:code => 'X2')
=> #<Country code: 2, name: nil>
>> Country.all
=> [#<Country code: "ZW", name: "Zimbabwe">, #<Country code: nil, name: nil>, #<Country code: "X1", name: nil>]

After "set_primary_key" is defined, Country.create no longer works. The :code field (primary key) of newly created records is a nil. find_or_create_by_code still works.