CarrierWaveでデフォルト画像を設定できない

railsでgem「CarrierWave」を使用してユーザー画像のアップロード処理を行なっています。

そこで、userが画像をアップロードしなかったら(user_imageがnil）だったら、デフォルト画像を表示させるように設定したのですが、以下のエラーが発生してしまいます。

Nil location provided. Can't build URI.

#エラー発生部分
<%= image_tag @current_user.user_image.url, class: 'user_image' %>

CarrierWaveでのデフォルト画像設定は以下のコードです。どこが不備があるのでしょうか?ご教授願います。

default.jpgの配置場所は、aap/assets/images　にあります。

class UserImageUploader < CarrierWave::Uploader::Base
  # Include RMagick or MiniMagick support:
  # include CarrierWave::RMagick
  # include CarrierWave::MiniMagick

  # 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

  # Provide a default URL as a default if there hasn't been a file uploaded:
  def default_url(*args)
  #   # For Rails 3.1+ asset pipeline compatibility:
  #   #ActionController::Base.helpers.asset_path("fallback/" + [version_name, "default.jpg"].compact.join('_'))
    "default.jpg"
  #  "/images/" + [version_name, "default.png"].compact.join('_')
  end

  # Process files as they are uploaded:
  # process scale: [200, 300]
  #
  # def scale(width, height)
  #   # do something
  # end

  # Create different versions of your uploaded files:
  # version :thumb do
  #   process resize_to_fit: [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_whitelist
  #   %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
  #   "something.jpg" if original_filename
  # end
end

  def create
    @user = User.new(user_params)
    if @user.save
      session[:user_id] = @user.id
      flash[:notice] = "ユーザー登録が完了しました"
      redirect_to posts_url
    else
      render 'new'
    end
  end

  def user_params
    params.require(:user).permit(:name,:password,:password_confirmation,:user_image)
  end

<%= form_with model: @user,local: true do |f| %>

      <%= f.file_field :user_image %>
      <%= f.submit "登録", class: "new-submit" %>

<% end %>