Unexpected behaviour of named_scope with :conditions and :include options

When I specify both conditions and a include for a named scope, the conditions are in at least some cases applied to the included relation as well. This behavior seems strange to me. I'll give an example:

class Category < ActiveRecord::Base

  belongs_to :parent, :class_name => 'Category'

  has_many :children, :class_name => 'Category', :foreign_key => :parent_id

  named_scope :root_categories, :conditions => {:parent_id => nil}, :include => :children

end

In this example Category.root_categories will return all root categories but calling the children method on any of these will result in an empty array, even though children for these records exist in the database (when I omit the include option children are returned). The problem became apparent from looking at the log, which showed that AR had generated the following query to fetch the include relation:

SELECT `categories`.* 
FROM `categories` 
WHERE (`categories`.parent_id IN (130,180,955, etc. etc.)) 
AND (`categories`.`parent_id` IS NULL) 
ORDER BY name;