acts_as_nested_set *** ArgumentError Exception: Unknown key: :order

Upgrading rails3.2.21 to 4.1.9 and in a class i have use acts_as_nested_set and there is a has_many relation between two class - see below code

class Area < ActiveRecord::Base
    acts_as_nested_set
    has_many :plans
end

class Plan < ActiveRecord::Base
     belongs_to :area
end

when i am try to calculate

p = Plan.first

p.area

through an error on terminal like-

ActionView::Template::Error (Unknown key: :order. Valid keys are: :class_name, :class, :foreign_key, :validate, :autosave, :table_name, :before_add, :after_add, :before_remove, :after_remove, :extend, :primary_key, :dependent, :as, :through, :source, :source_type, :inverse_of, :counter_cache, :join_table):

how to fix it?