threads.js: Uncaught TypeError: importScripts.map is not a function(…)

I’m trying to use threads.js do some some processing but am encountering this error:

Uncaught TypeError: importScripts.map is not a function(…)

The workflow goes like this. I first import Pool from the threads module:

import {Pool} from 'threads';

And then call it as in the documentation:

 let jobA = this.threadPool.run(function(input, done) {
                workerGetTilesetInfo(input);
                done(input);
            }, {
                workerGetTilesetInfo: 'worker'
            })

           .on('done', function(job, message) {
                console.log('done', message);
           })
           .on('error', function(job, error) {
                console.log('error', error);
           })
            .send(outUrl);

The function getTilesetInfo is defined in another javascript file called worker.js. Is this a valid issue, or am I simply doing something wrong?