Create disk using custom image

Using Node.js google cloud compute, how can i create a disk using a custom image?

The documentation shows the following, but I can't find any mention of custom images that work:

zone.createDisk('disk-name', {
  sourceImage: 'my-custom-image',
  sizeGb: 20,
}, callback);

This results in

ApiError: Invalid value for field 'resource.sourceImage'

As with most Google APIs I usually learn from reading the tests, but I don't see any tests using images.

via d-_-b