Gulp TypeError: Cannot read property 'apply' of undefined

Running any type of Gulp command gives me the following error:

TypeError: Cannot read property ‘apply’ of undefined

I’ve tried doing a yarn upgrade --latest, but it didn’t help.

What theme are you working on, and what version of node are you using?

Casper w/ Node 10.15.3

Do you have any other output? What command are you running?

It looks like it only happens when I run gulp build directly. Running yarn pretest works though. User error, sorry.

When running yarn dev, the page doesn’t live-reload on change. How can I enable that?