Problem deserialising attribute between 1.9.2-p290 and p180

Hello,
    In the process of updating from 1.9.2-p180 to 1.9.2-p290 I found
that deserialising a serialised attribute as a Hash, which contains
other models, stopped working. If I reference the models which are
included in the hash prior to deseralising everything works fine.

    Instantiating the model in p290 without referencing the models
raises:
ActiveRecord::SerializationTypeMismatch: combination was supposed to
be a Hash, but was a String

    This is on Rails 3.0.10, the database field holding the attribute
is large enough to hold the serialised attribute. The models in the
hash are Price and a bunch of Shop models.
    RVM is used to switch between p180 and p290. The gem versions are
identical (gemset was copied from p180 to p290).

class ListPrice < ActiveRecord::Base
  ## Referencing the models before deserialising the attribute makes
deserialising work in 1.9.2-p290, but is not needed in 1.9.2-p180
  Price
  Shop.all.each {|shop| shop}
  serialize :combination, Hash

...
end

Any thoughts on what's happening here?

Cheers,
Dan