Rails 2.3.4 ActiveRecord association problem

Upgraded our app from Rails 2.2.2 to 2.3.4, and I'm now encountering
an error when adding a child to a parent.

Top of the stack:

wrong number of arguments (1 for 0)
/Library/Ruby/Gems/1.8/gems/activerecord-2.3.4/lib/active_record/
associations/has_many_association.rb:61:in `save'
/Library/Ruby/Gems/1.8/gems/activerecord-2.3.4/lib/active_record/
associations/has_many_association.rb:61:in `insert_record'
/Library/Ruby/Gems/1.8/gems/activerecord-2.3.4/lib/active_record/
associations/association_collection.rb:119:in `<<'
/Library/Ruby/Gems/1.8/gems/activerecord-2.3.4/lib/active_record/
associations/association_collection.rb:433:in
`add_record_to_target_with_callbacks'

My code (in snippets):

  class Trip < ActiveRecord::Base
    has_many :activity_reservations
  end

  class ActivityReservation < ActiveRecord::Base
    belongs_to :trip
  end

  trip.activity_reservations << ActivityReservation.new

The has_many_associations code referenced:

        def insert_record(record, force = false, validate = true)
          set_belongs_to_association_for(record)
          force ? record.save! : record.save(validate)
        end

The ActiveRecord save method:

      def save
        create_or_update
      end

The comments for the save method indicate that it can take an optional
perform_validations method, and the Validations module defines (with
alias_method_chain :save, :validation).

So, finally my question: what is going on here? ActiveRecord is
calling a save method passing an arg, but the save method in
ActiveRecord isn't defined to take a boolean. I'm guessing that
something with the Validations and alias_method_chain is supposed to
handle this, but in the way I'm doing things in my code, its not
working.

(ActiveRecord 2.2.2, which we were on previously, just called
ActiveRecord.save - not attempting to pass a boolean)

Any help or pointers would be greatly appreciated.

Paul Christmann