File size validator- presence true not working

song.rb

require 'file_size_validator'
class Song < ActiveRecord::Base
  mount_uploader :attachment, AttachmentUploader

  validates :attachment, :presence => true, 
  			:file_size => {:maximum => 2.megabytes.to_i}
end

when I submit form without file or empty form it gives error :

ActionController::ParameterMissing at /songs

param is missing or the value is empty: song
- since I'm new to rails does validates :presence => true not working or the problem is any thing else.