Running the migration from page 126 throws an error

[lifeBalance]

When I try to run this migration:

class AddIndexToLinks < ActiveRecord::Migration
  def change
    add_index :links, :cl_sortkey
  end
end

I get the following error:

$ bundle exec rake db:migrate
== 20151121175520 AddIndexToLinks: migrating ==================================
-- add_index(:links, :cl_sortkey)
rake aborted!
StandardError: An error has occurred, all later migrations canceled:

Mysql2::Error: BLOB/TEXT column 'cl_sortkey' used in key specification without a key length: CREATE  INDEX `index_links_on_cl_sortkey`  ON `links` (`cl_sortkey`) /Users/javi/.gem/ruby/2.2.2/gems/mysql2-0.4.1/lib/mysql2/client.rb:85:in `_query'

Maybe has something to do with having populated my db from the seeds file, instead of the dumps?