threads.js: DataCloneError: Failed to execute 'postMessage' on 'Window': ErrorEvent object could not be cloned.

I’m getting this error reported in Sentry:

Timeout: Did not receive an init message from worker after 10000ms. Make sure the worker calls expose().

With another error:

Not reporting uncaught error back to master thread as it occured while reporting an uncaught error already. Latest error: DataCloneError: Failed to execute 'postMessage' on 'Window': ErrorEvent object could not be cloned.

The second error might be unrelated, it’s hard to see in Sentry. But it seems to happen 9 or 10 seconds before the timeout error so that matches up.

Seems to happen to both Chrome 80 and Safari 13 users.

Any idea what might be causing this? My workers all call expose():

import { expose } from 'threads/worker'

...

 expose({
    getCommonLocations
  })