awesome_nested_set: Impossible move, target node cannot be inside moved tree

[sharp]

#encoding: utf-8
class Grade < ActiveRecord::Base
  acts_as_nested_set
  attr_accessible :name, :parent_id
  has_many :subjects
  scope :stages, where("parent_id is null")
  scope :list, where("parent_id <> ''")
  #scope :excluding_ids, lambda { |ids| where(['id NOT IN (?)', ids]) if ids.any?}

end

This my seeds

puts "start import "
stage = Grade.create :name => "grade"
["grade1", "grade2"].each do |name|
  grade = Grade.create :name => name, :parent_id => stage.id
end

When I run rake db:seed, it shows

rake aborted! Impossible move, target node cannot be inside moved tree.

[sharp]

I just type the wrong column: rgt -> rft

[lucasprag]

For me I had corrupted data, so I did this: Model.rebuild!