Default URL not downloadable with Carrierwave in Rails

I'm having problems setting the default image in my carrier-supported bootloader. It seems to add a weird class to the top of the url, but it doesn't create an image. See my code below.

Assistant

 # encoding: utf-8

class UserpicUploader < CarrierWave::Uploader::Base

  CarrierWave::SanitizedFile.sanitize_regexp = /[^[:word:]\.\-\+]/
  # Include RMagick or MiniMagick support:
  include CarrierWave::RMagick
  # include CarrierWave::MiniMagick
  # Include the Sprokets helpers for Rails 3.1+ asset pipeline compatibility:
  # include Sprockets::Helpers::RailsHelper
  # include Sprockets::Helpers::IsolatedHelper

  # Choose what kind of storage to use for this uploader:
  storage :file
  # storage :fog

  # Override the directory where uploaded files will be stored.
  # This is a sensible default for uploaders that are meant to be mounted:
  def store_dir
    "uploads/#{model.class.to_s.underscore}/#{mounted_as}/#{model.id}"
  end



  # Process files as they are uploaded:

  version :normal do
   process :resize_to_fill => [162, 163]
  end
  # Provide a default URL as a default if there hasn't been a file uploaded:

     def default_url
     # For Rails 3.1+ asset pipeline compatibility:
       # asset_path("fallback/" + [normal, "profile_default_pic.png"].compact.join('_'))
       "/images/fallback/" + [normal, "profile_default_pic.png"].compact.join('_')
     end

  #
  # def scale(width, height)
  #   # do something
  # end

  # Create different versions of your uploaded files:
  # version :thumb do
  #   process :scale => [50, 50]
  # end

  # Add a white list of extensions which are allowed to be uploaded.
  # For images you might use something like this:
  def extension_white_list
     %w(jpg jpeg gif png)
  end

  # Override the filename of the uploaded files:
  # Avoid using model.id or version_name here, see uploader/store.rb for details.
    def filename 
      if original_filename 
        @name ||= Digest::MD5.hexdigest(File.dirname(current_path))
        "#{@name}.#{file.extension}"
      end
    end

end

EDIT ...

Here is the edited code that worked. I had to put the image in open / backup, and not in Assets / images / fallback.

# encoding: utf-8

class UserpicUploader < CarrierWave::Uploader::Base

  CarrierWave::SanitizedFile.sanitize_regexp = /[^[:word:]\.\-\+]/
  # Include RMagick or MiniMagick support:
  include CarrierWave::RMagick
  # include CarrierWave::MiniMagick
  # Include the Sprokets helpers for Rails 3.1+ asset pipeline compatibility:
  include Sprockets::Helpers::RailsHelper
  include Sprockets::Helpers::IsolatedHelper

  # Choose what kind of storage to use for this uploader:
  storage :file
  # storage :fog

  # Override the directory where uploaded files will be stored.
  # This is a sensible default for uploaders that are meant to be mounted:
  def store_dir
    "uploads/#{model.class.to_s.underscore}/#{mounted_as}/#{model.id}"
  end



  # Process files as they are uploaded:

  version :normal do
   process :resize_to_fill => [162, 163]
  end
  # Provide a default URL as a default if there hasn't been a file uploaded:

     def default_url
     # For Rails 3.1+ asset pipeline compatibility:
         asset_path("fallback/" + [normal, "profile_default_pic.png"].compact.join('_'))
       # "/fallback/" + [normal, "profile_default_pic.png"].compact.join('_')
     end

  #
  # def scale(width, height)
  #   # do something
  # end

  # Create different versions of your uploaded files:
  # version :thumb do
  #   process :scale => [50, 50]
  # end

  # Add a white list of extensions which are allowed to be uploaded.
  # For images you might use something like this:
  def extension_white_list
     %w(jpg jpeg gif png)
  end

  # Override the filename of the uploaded files:
  # Avoid using model.id or version_name here, see uploader/store.rb for details.
    def filename 
      if original_filename 
        @name ||= Digest::MD5.hexdigest(File.dirname(current_path))
        "#{@name}.#{file.extension}"
      end
    end

end