Error for update model with attachment_fu without file

class AlbumSong < ActiveRecord::Base

  MAX_SIZE_IN_MB = 10

  has_attachment :content_type => 'audio/mp3',     :storage => :s3,     :max_size => MAX_SIZE_IN_MB.megabytes

  acts_as_indexed :fields => [:title, :author],     :index_file => [Rails.root.to_s, "tmp", "index"]

  belongs_to :album

  validates_presence_of :title   validates_uniqueness_of :title end

When update a model without file, it appears the following error. How can I solve?

TypeError: can't convert nil into String         from /home/marcelo/hueb/bandas/vendor/plugins/attachment_fu/lib/technoweenie/attachment_fu/backends/s3_backend.rb:262:in `join'         from /home/marcelo/hueb/bandas/vendor/plugins/attachment_fu/lib/technoweenie/attachment_fu/backends/s3_backend.rb:262:in `full_filename'         from /home/marcelo/hueb/bandas/vendor/plugins/attachment_fu/lib/technoweenie/attachment_fu.rb:367:in `temp_paths'         from /home/marcelo/hueb/bandas/vendor/plugins/attachment_fu/lib/technoweenie/attachment_fu.rb:361:in `temp_path'         from /home/marcelo/hueb/bandas/vendor/plugins/attachment_fu/lib/technoweenie/attachment_fu.rb:320:in `save_attachment?'         from /home/marcelo/hueb/bandas/vendor/plugins/attachment_fu/lib/technoweenie/attachment_fu.rb:424:in `set_size_from_temp_path'         from /usr/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/callbacks.rb:178:in `send'         from /usr/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/callbacks.rb:178:in `evaluate_method'         from /usr/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/callbacks.rb:166:in `call'         from /usr/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/callbacks.rb:93:in `run'         from /usr/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/callbacks.rb:92:in `each'         from /usr/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/callbacks.rb:92:in `send'         from /usr/lib/ruby/gems/1.8/gems/activesupport-2.3.5/lib/active_support/callbacks.rb:92:in `run'         from /home/marcelo/hueb/bandas/vendor/plugins/attachment_fu/lib/technoweenie/attachment_fu.rb:492:in `run_callbacks'         from /usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/callbacks.rb:344:in `callback'         from /usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/callbacks.rb:311:in `valid?'         from /usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/validations.rb:1077:in `save_without_dirty'         from /usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/dirty.rb:79:in `save_without_transactions'         from /usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/transactions.rb:229:in `send'         from /usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/transactions.rb:229:in `with_transaction_returning_status'         from /usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/connection_adapters/abstract/database_statements.rb:136:in `transaction'         from /usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/transactions.rb:182:in `transaction'         from /usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/transactions.rb:228:in `with_transaction_returning_status'         from /usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/transactions.rb:196:in `save'         from /usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/transactions.rb:208:in `rollback_active_record_state!'         from /usr/lib/ruby/gems/1.8/gems/activerecord-2.3.5/lib/active_record/transactions.rb:196:in `save'         from (irb):4>>