Error when trying to build in antora-default-ui

I've added this extension to my antora-default-ui by adding the following in ui-model.yml (and installing it via npm)

asciidoc:
  extensions:
  - '@djencks/asciidoctor-mathjax'

But when I run gulp preview, I get the following error

TypeError: Cannot read properties of undefined (reading 'register')
    at module.exports.register (/Users/REDACTED/node_modules/@djencks/asciidoctor-mathjax/lib/asciidoctor-mathjax.js:69:23)
    at /Users/REDACTED//docs/gulp.d/tasks/build-preview-pages.js:28:28
    at Array.map (<anonymous>)
    at /Users/REDACTED//gulp.d/tasks/build-preview-pages.js:25:74
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)