undefined method `extname' for #<Jekyll::Layout:

[shivabhusal]

trafficstars

 $ jekyll s

Configuration file: /Users/john/projects/csheet/_config.yml
            Source: /Users/john/projects/csheet
       Destination: /Users/john/projects/csheet/_site
 Incremental build: disabled. Enable with --incremental
      Generating... 
jekyll 3.8.5 | Error:  undefined method `extname' for #<Jekyll::Layout:0x00007f877bd17220>

this happened when I added following in my Gemfile

group :jekyll_plugins do
  gem 'jekyll-slim'
  gem 'slim', github: 'slim-template/slim'
end

[ashesh-banbagroup]

I faced the same problem. Can anyone help with this?

[activestylus]

Same here. Not working at all on v3.8.6