sprockets 4 - error in C function image-url

[michaelglass]

Env: Ubuntu 16.04
Ruby: 2.5.3
sprockets: 4.0.0-beta8
sassc: 2.0.1
sassc-rails: 2.1.1
rails: 5.2.2.1

Upgrading to sprockets 4, I started to see this error:

(longer error here)

08:39:00  2019-04-25 06:39:00 UTC	INFO	      2.54 ms processing(#<SassC::Rails::SassTemplate:0x0000557cc00b91d0>,text/css) -> app/assets/stylesheets/values.sass
08:39:01  2019-04-25 06:39:00 UTC	INFO	      0.06 ms processing(#<Sprockets::Preprocessors::Defarake aborted!
08:39:01  SassC::SyntaxError: Error: error in C function image-url: undefined method `[]' for nil:NilClass
08:39:01          on line 1794 of vendor/assets/stylesheets/bootstrap.min.scss, in function `image-url`
08:39:01          from line 1794 of vendor/assets/stylesheets/bootstrap.min.scss
08:39:01          from line 3 of app/assets/stylesheets/content_creation/base.sass
08:39:01          from line 1 of app/assets/stylesheets/content_creation/template-builder.sass
08:39:01  >>     background-image: image-url("bootstrap/img/glyphicons-halflings.png");
08:39:01  
08:39:01     ----------------------^
08:39:01  /home/jenkins-node/workspace/NoRedInk_sprockets-percy/vendor/assets/stylesheets/bootstrap.min.scss:1794

Any idea how I could start troubleshooting this?

[michaelglass]

fixed by disabling concurrency, see: rails/sprockets#581 (comment)