Undefined method updated? for <model object>

Hi,
I have two model say-

ZipCode < ActiveRecord::Base
belongs_to :state_provinces
end

StateProvince < ActiveRecord::Base
has_many :zip_codes
end

if I do @zip_code.update_atribute(:city, “My City”)
its giving Undefined method updated? for StateProvince

if I remove belongs_to specifier in ZipCode class then it work fine.
Any alternate solution??

Thanks

Could you provide the error trace?

Cheers,
Nicholas