How to make Local Activities with TypeScript SDK

I have some workflows and activities that work perfectly fine.

I want to make the activities local activities (Activities | Temporal Documentation) to decrease load on the temporal cluster.

Looking through the docs, it seems I should be able to achieve this by replacing
const { f } = proxyActivities<typeof activities>(activityOptions);
const { f } = proxyLocalActivities<typeof activities>(activityOptions);

But when I trigger the new workflows, the worker fails and shuts down with this message:

Temporal worker error
Error: Expected timestamp, got null
    at tsToMs (/Users/richard.tweeddale/code/node_modules/@temporalio/common/src/time.ts:30:11)
    at extractActivityInfo (/Users/richard.tweeddale/code/node_modules/@temporalio/worker/src/worker.ts:1820:34)
    at async /Users/richard.tweeddale/code/node_modules/@temporalio/worker/src/worker.ts:849:34
    at async /Users/richard.tweeddale/code/node_modules/@temporalio/worker/src/tracing.ts:43:14
    at async /Users/richard.tweeddale/code/node_modules/@temporalio/worker/src/worker.ts:829:22

It works fine when I use proxyActivities instead of proxyLocalActivities.

Are there other steps needed to make activities into local activities?

That seems off.
Which SDK version are you using?
Can you provide a minimal reproduction?

@bergundy See related Slack discussion.