fetch client with throwErrors off: how to disambiguate union?

[jamesarosen]

The fetch client example has this code:

const { data, error } = await addPet({
  // ...
})
if (error) {
  console.log(error);
  return;
}
setPet(data!);

but when I try to do that, I get a TypeScript error on the destructuring assignment:

Property 'data' does not exist on type 'Promise<({ data: undefined; error: unknown; } | { data: Pet; error: undefined; }) & { request: Request; response: Response; }>'.ts(2339)

I would expect there to be a type-narrowing function like

import { isError, isSuccess } from '@hey-api/openapi-ts'
const result = await addPet({...})
if (isError(result)) {
  console.log(result.error);
  return;
}
if (isSuccess(result)) {
  setPet(result.data);
}

but I don't see anything like that in the docs.

I'm using TypeScript@5.8.3. I have "strictNullChecks": false in my TSConfig, which I suspect might be the problem.