has_many :through generates bad SQL during include?

If I have a class that has_many of another class :through a join table, I find I get a SQL error from the .include?(obj) method. This doesn't happen if the related data is already loaded.

class Channel < ActiveRecord::Base

has_many :users, :through => :subscriptions

end

u = User.find(:first)

c = Channel.find(:first)

c.users.include?(u)

Mysql::Error: Column 'id' in where clause is ambiguous: SELECT `users`.id FROM `users1` INNER JOIN subscriptions ON users.id = subscriptions.user_id WHERE (id = 19) AND ((`subscriptions`.channel_id = 54)) LIMIT 1

>c.users

>c.users.include?(u)

true

This code worked fine before upgrading to 2.1. I can't seem to find where this bare "id" in the WHERE clause could be coming from...