Can’t run a migration adding userstamp to an existing table Rails

Looking at https://github.com/infused/blamer , by default the two columns are created_by and updated_by with type integer. So…

add_column :illustrators, :created_by, :integer
add_column :illustrators, :updated_by, :integer

(integer being used in https://github.com/infused/blamer/blob/master/lib/blamer/userstamp.rb#L47 )