bigint primary keys break sequel

$ psql
=# create table bigtable ( id bigserial, name text );
NOTICE:  CREATE TABLE will create implicit sequence "bigtable_id_seq" for serial column "bigtable.id"
CREATE TABLE
=# \d bigtable
                         Table "public.bigtable"
 Column |  Type  |                       Modifiers                       
--------+--------+-------------------------------------------------------
 id     | bigint | not null default nextval('bigtable_id_seq'::regclass)
 name   | text   | 
=# \q

$ sequel -d postgres://postgres@localhost/taps
Class.new(Sequel::Migration) do
  def up
    create_table(:bigtable) do
      Bignum :id, :null=>false
      String :name, :text=>true
    end
  end
  
  def down
    drop_table(:bigtable)
  end
end