ActiveRecord::StatementInvalid in ImageboardController#index

Mysql::Error: #23000Column 'parent_id' in where clause is ambiguous: SELECT COUNT(DISTINCT comments.id) FROM comments  LEFT OUTER JOIN comments childrens_comments ON childrens_comments.parent_id = comments.id WHERE (parent_id IS NULL && board_id = 1)

class ImageboardController < ApplicationController
  def index
    @pages, @parents =
      paginate( :comments,
                :include => [:children],
                :per_page => @board.setting(:threads_per_page),
                :conditions => "parent_id IS NULL && board_id = #{@board.id}",
                :order => "bumped_on DESC")
  end
end

require 'cgi'
require 'digest/md5'
require 'RMagick'
include Magick

class Comment < ActiveRecord::Base
  belongs_to    :board
  acts_as_tree  :order => "created_on ASC", :dependent => :destroy
end