Union type tuple destruction not working on TS

index.ts

const returnError = async <R>(callback: (...args: any) => Promise<R>): Promise<[R] | [undefined, Error]> => {
  try {
    return await callback()
  } catch (e) {
    return e
  }
}
  
const [result, err] = returnError(
  () => api.call(/* ... */)
)

if (err instanceof Error) {
  // . some logic and return / throw
}

result.someValue // Problem: TS will still show, that result can be undefined :(