jekyll-webmention_io: jekyll webmention command cannot be found

After installing I have been able to get the plugin working as intended. However, the jekyll webmention command fails with the following error:

fatal: 'jekyll webmention' could not be found. You may need to install the jekyll-webmention gem or a related gem to be able to use this subcommand.

The error doesn’t make much sense as the plugin working for other things.