"clock should extend target type" error while logically correct forwarding

[yumauri]

This code

const a = createEvent<{ a: any }>()
const b = createEvent<{ b?: any }>()

sample({
  clock: a,
  target: b
})

complains

Argument of type '{ clock: Event<{ a: any; }>; target: Event<{ b?: any; }>; }' is not assignable to parameter of type '{ error: "clock should extend target type"; targets: { clockType: { a: any; }; targetType: { b?: any; }; }; }'.
  Object literal may only specify known properties, and 'clock' does not exist in type '{ error: "clock should extend target type"; targets: { clockType: { a: any; }; targetType: { b?: any; }; }; }'.(2345)

I think it is incorrect, because logically it should be absolutely fine to forward { a: any } to { b?: any }. Correct me if I am wrong.
Same issue happens with sample, split, forward and maybe other methods too.