Carrierwave Backgrounder in Rails 5.2?

I have an image uploader on a form, and images upload fine without the background processing. Since moving to Heroku, the larger source images often time out, so I'm looking to move this to a background job. In doing so, I looked at carrierwave_backgrounder since it seemed to cover what I need.

I've found a fork of carrierwave_backgrounder and I've forked it to my own repo. To get it working in 5.2, the author said he had to hardcode the path (https://github.com/lardawge/carrierwave_backgrounder/issues/282, https://github.com/lardawge/carrierwave_backgrounder/issues/280)

This fails on store_in_background. Running just process_in_background yields locally processed images in development, but they are not uploaded to S3.

The field in quesiton is issue.image and here's how it's mounted in issue.rb:

# Image attachment via Carrierwave
  mount_uploader :image, ImageUploader
  process_in_background :image
  store_in_background :image
  attr_accessor :image_cache

Here's the output error:

2019-02-24T01:05:33.168Z 43666 TID-oxnydp4bq CarrierWave::Workers::StoreAsset JID-83163770b014d37186118cf9 INFO: start
2019-02-24T01:05:33.545Z 43666 TID-oxnydp4bq CarrierWave::Workers::StoreAsset JID-83163770b014d37186118cf9 INFO: fail: 0.377 sec
2019-02-24T01:05:33.545Z 43666 TID-oxnydp4bq WARN: {"context":"Job raised exception","job":{"class":"CarrierWave::Workers::StoreAsset","args":["Issue","6355","image"],"queue":"carrierwave","retry":true,"jid":"83163770b014d37186118cf9","created_at":1550970333.167957,"enqueued_at":1550970333.168011},"jobstr":"{\"class\":\"CarrierWave::Workers::StoreAsset\",\"args\":[\"Issue\",\"6355\",\"image\"],\"queue\":\"carrierwave\",\"retry\":true,\"jid\":\"83163770b014d37186118cf9\",\"created_at\":1550970333.167957,\"enqueued_at\":1550970333.168011}"}
2019-02-24T01:05:33.545Z 43666 TID-oxnydp4bq WARN: TypeError: no implicit conversion of nil into String
2019-02-24T01:05:33.545Z 43666 TID-oxnydp4bq WARN: /Users/jathayde/Development/Meticulous/carrierwave_backgrounder/lib/backgrounder/workers/store_asset_mixin.rb:40:in `join'

and here's the relevant method from my store_asset_mixin.rb method from my local version of the gem. The line 40 mentioned in the error is the one that starts with @tmp_directory:

      def store_directories(record)
        asset, asset_tmp = record.send(:"#{column}"), record.send(:"#{column}_tmp")
        cache_directory  = File.expand_path(asset.cache_dir, asset.root)
        # @cache_path      = File.join(cache_directory, asset_tmp)
        # # XXX Hardcoded our path here... not ideal..
        @cache_path      = open("https://patchvault.s3.amazonaws.com/uploads/tmp/#{asset_tmp}")
        @tmp_directory   = File.join(cache_directory, asset_tmp.split("/").first)
      end

So asset_tmp is "", yet asset but I have no idea why. Stepping thorugh it with debugger it seems that the record.image is present, but the record.image_tmp column is simply empty.