Sequel migration ran in heroku yields postgres type error

For some reason when I run my migrations in Heroku Sequel tries to create a row as string instead of varchar:

PGError: ERROR:  type "string" does not exist (Sequel::DatabaseError)
LINE 1: ...erial PRIMARY KEY, "开发者_Python百科facebook_id" integer, "email" string(50)

Anyone has a fix?

---

Ok changed my migration from:

string  :email,                   :null => false, :unique => true

to

String  :email,                   :null => false, :unique => true