rails: Uniq has_many :through with default_scope order does not work with PostgreSQL

Given these models:

class Tracker < ActiveRecord::Base
  has_many :issues
  has_many :categories, :through => :issues, :uniq => true
end

class Issue < ActiveRecord::Base
  belongs_to :tracker
  belongs_to :category
  default_scope order(:number)
end

class Category < ActiveRecord::Base
  has_many :issues
  default_scope order(:name)
end

When calling tracker.categories, on PostgreSQL, this fails with the following error:

ActiveRecord::StatementInvalid: PG::Error: ERROR:  for SELECT DISTINCT, ORDER BY expressions must appear in select list

SELECT DISTINCT "categories".* FROM "categories" INNER JOIN "issues" ON "categories"."id" = "issue"."category_id" WHERE "issue"."tracker_id" = 42 ORDER BY number, name