default_scope prevents a rake db:migrate if attributes in DB don't exist

Rails version 3 beta 4

I just discovered that if I declared the following in the User model for example:

default_scope :conditions => { :cancelled_at => nil }

and run rake db:migrate on an empty database I get:

rake aborted!
No attribute named `cancelled_at` exists for table `users`

The following code gets around the problem, but it's not pretty.

default_scope do 
  { :conditions => { :cancelled_at => nil } }
end