How to include an external class into an arktype type?

import { TimeStub } from 'fauna'; // class

const clientDocument = type({
    'id?': 'string',
    'coll?': 'string',
    'ts?': TimeStub
});

import { TimeStub } from 'fauna'; // class

const clientDocument = type({
    'id?': 'string',
    'coll?': 'string',
    'ts?': ['instanceof', TimeStub]
});