Attachement_fu error when updating existing photos

Hi,

I am attempting to replace an exiting photo with a new one uploaded by
the user. I am using attachment_fu and I am coming up with the
following exception:

can't convert nil into String

/vendor/plugins/attachment_fu/lib/technoweenie/attachment_fu/backends/file_system_backend.rb:23:in
`join'
/vendor/plugins/attachment_fu/lib/technoweenie/attachment_fu/backends/file_system_backend.rb:23:in
`full_filename'
/vendor/plugins/attachment_fu/lib/technoweenie/attachment_fu.rb:369:in
`temp_paths'
/vendor/plugins/attachment_fu/lib/technoweenie/attachment_fu.rb:354:in
`uploaded_data='

My ruby code:
<% form_for(@photo) do |f| %>
  <%= f.error_messages %>

  <%= image_tag(@photo.public_filename, :BORDER=>0) %>

  <p>
  <label for="upload">Upload new photo:</label>
    <%= f.file_field :uploaded_data %>
    <%= f.submit 'Update' %>
  </p>
<% end %>

My rails code:
def edit
  @photo = Photo.find(params[:id])
end

def update
  @photo = Photo.find(params[:id])

if @photo.update_attributes(params[:photo])
      flash[:notice] = 'Photo was successfully updated.'
      redirect_to('/listingHome/active/edit/' + @photo.lisitng_id)
end
end

Method failing in attachement_fu

# Gets the full path to the filename in this format: