cannot compare error using instanceof

[seoker]

Based on README, the error should be compared using instanceof operator.

for example

const errors = require('restify-errors');
const err = new errors.ResourceNotFoundError();
err instanceof errors.ResourceNotFoundError; // true

However I couldn't compare them in a specific case under restify:

// trigger this error when some in-existing route being called
server.on('NotFound', (req, res, err, cb) => {
  req.log.info(JSON.stringify(err, null, 2)); // { "code": "ResourceNotFound", "message": "/xxxx does not exist" }
  req.log.info(err.toString()); // ResourceNotFoundError: /xxxx does not exist
  req.log.info(err.code); // Error
  if (err instanceof errors.ResourceNotFoundError) {
    // never get here
  }

  cb();
});

kind of weird, any ideas?
I'm not sure where is better to report this issue, restify or this repo?

versions
restify: 5.2.0
restify-errors: 5.0.0

[seoker]

Had opened an issue at restify/restify#1496

[DonutEspresso]

Thanks for bringing this our attention! Let's keep the convo going at restify/node-restify#1496