Using a checksum to identify a unique directory

I have a rails app that compiles assets as part of the test suite. What was working before is no longer working:

      - checkout
      - run: mkdir -p tmp/test/cache/assets
      - attach_workspace:
          at: ~/fworkspace
      - run: tar -cf - app/assets | md5sum > asset_checksum.txt
      - restore_cache:
            - v4-app-assets-{{ checksum "asset_checksum.txt" }}
            - v4-app-assets-
      - run: npm install
      - run: bundle check --path=vendor/bundle || bundle install --path=vendor/bundle --jobs=4 --retry=3
      - run: bundle exec rake assets:precompile
      - save_cache:
          key: v4-app-assets-{{ checksum "asset_checksum.txt" }}
            - tmp/test/cache/assets
            - public/assets
      - persist_to_workspace:
          root: ~/fworkspace
            - tmp/test/cache/assets
            - public/assets

This used to check if there are any changes in the app/assets folder, and if not restore from cache, but if there are, go ahead and compile assets without restoring from cache.

I also tried a second suggestion found on the discussion boards:
- run: | echo $(find ./app/assets -type f -exec md5sum {} \; | md5sum | cut -d' ' -f1) >> ASSET_CACHE_KEY

This also does not see to generate the same key for the same app/assets. Any suggests what I may be doing wrong here?