Datamapper DataMapper → 1.0.0

I have such two models (among others):

class Activity
  include DataMapper::Resource
  property :id,          Serial
  property :invoice_id,  Integer, :index => true
  belongs_to :invoice
  ...
end

class Invoice
  include DataMapper::Resource
  property :id,          Serial
  has n, :activities
  ...
end

If I include dm-constraints, during the migration it tries to execute such SQL:

 ALTER TABLE `activities` ADD CONSTRAINT `activities_invoice_fk` FOREIGN KEY (`invoice_id`) REFERENCES `invoices` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION

And then I get this error:

Can't create table 'rubytime3_dev.#sql-a4_53' (errno: 150) (code: 1005, sql state: HY000, query: ALTER TABLE `activities` ADD CONSTRAINT `activities_invoice_fk` FOREIGN KEY (`invoice_id`) REFERENCES `invoices` (`id`) ON DELETE NO ACTION ON UPDATE NO ACTION, uri: mysql://root@localhostrubytime3_dev?encoding=UTF-8)

show innodb status in mysql console tells me that:

Cannot find an index in the referenced table where the
referenced columns appear as the first columns, or column types
in the table and the referenced table do not match for constraint.

I assume that the primary key (serial) is indexed by default, so what's the problem? Is it that invoices.id is created as INT(10) UNSIGNED NOT NULL AUTO_INCREMENT, and activities.invoice_id as INTEGER and these are different data types? In this case, am I doing something wrong or is DataMapper creating the columns incorrectly?