Help troubleshooting jekyll-compose plugin

I have installed the plugin jekyll-compose, following the instructions on the website.

However, I am getting a curious error when I try to use it:

$ bundle exec jekyll post "New Post"
Configuration file: /Users/username/Website/blog/_config.yml
jekyll 3.4.0 | Error:  no implicit conversion of nil into String

What process can I use to troubleshoot this error?