`E.showPrompt` not working in emulator?

I'm trying to do development and have hit a brick wall with a strange problem I can't diagnose. E.showPrompt works fine on my physical Bangle.js, but it fails with an incomprehensible error message when I try to use it in an emulator. Even after clearing all of the persistent in-browser data for the Espruino Web IDE and starting on a clean slate, the following simple example doesn't work:

E.showPrompt('foo', {
  title: 'bar',
  buttons: { "A": true, "B": false }
}).then(function (arg) {
  console.log('test');
});

When I click a button, nothing inside the then() gets executed at all, and instead I get this console message:

Uncaught Error: Unknown argspec 33352

What does that even mean? I grepped the entire project directory and came up with no code matches for “argspec” at all.

It works if I send it to my actual Bangle, but developing it that way right now is quite literally a pain in the neck—my vision isn't very good. I'd rather stick with the emulator most of the time.