no implicit conversion of nil into String carrierwave

carrierwave で、動画投稿機能を作成していました。
最初の内は上手く動いていたのですが、
bundle update した後より

no implicit conversion of nil into String

が表示されるようになりました。

carrierwave-version: carrierwave (0.10.0)

      - if post.video.present?
         .img-responsive.well alt="Responsive image"
           = link_to image_tag(post.video_url(:screenshot)), post
       - if post.body.present?
         p.post_text.well

Application Log

app/uploaders/video_uploader.rb:36:in `dirname'
app/uploaders/video_uploader.rb:36:in `screenshot'
app/views/posts/index.html.slim:37:in `block in _app_views_posts_index_html_slim__885061408226101170_70165504801060'
app/views/posts/index.html.slim:23:in `_app_views_posts_index_html_slim__885061408226101170_70165504801060'

app/uploaders/video_uploader.rb

require 'streamio-ffmpeg'
require 'carrierwave'
class VideoUploader < CarrierWave::Uploader::Base
  storage :file

  def store_dir
    "uploads/#{model.class.to_s.underscore}/#{mounted_as}/#{model.id}"
  end
  # Add a white list of extensions which are allowed to be uploaded.
  # For images you might use something like this:

  # 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 mov wmv mp4 flv avi)
  end

  version :screenshot do
    process :screenshot
    def full_filename (for_file = model.logo.file)
      "screenshot.jpg"
    end
  end

  def screenshot
    tmpfile = File.join(File.dirname(current_path), "tmpfile")
    File.rename(current_path, tmpfile)
    movie = FFMPEG::Movie.new(tmpfile)
    movie.screenshot(current_path + ".jpg", preserve_aspect_ratio: :width)
    File.rename(current_path + ".jpg", current_path)
    File.delete(tmpfile)
  end
end