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>>