Rails 4无法转储表“users”，因为跟随NoMethodError #nurfined方法`[]'为nil：NilClass

Question

当我在下面运行此迁移时，我开始在schema.rb文件中收到错误。我正在关注UUID (tutorial)的教程，我不确定这是否正确。我有它工作正常，但我无法看到schema.rb文件中的表，我无法运行rake db：rollback。

 class AddUuidToUsers < ActiveRecord::Migration
  def up
   add_column :users, :uuid, :uuid
  end

 def down
   remove_column :users, :uuid
 end
end

这是我运行rake db：rollback

时的错误

undefined method `to_sym' for    nil:NilClass/Applications/MAMP/htdocs/Sites/ubergradd/db/migrate/20141024175231_add_uuid_to_users.rb:7:in `down'
NoMethodError: undefined method `to_sym' for nil:NilClass
/Applications/MAMP/htdocs/Sites/ubergradd/db/migrate/20141024175231_add_uuid_to_users.rb:7:in `down'

在我的schema.rb文件中

 # Could not dump table "users" because of following NoMethodError
 #   undefined method `[]' for nil:NilClass