Empty object not of exact type with optional properties

Minimal example:

``` js
type PossiblyEmpty = $Exact<{
  a?: number
}>

const p: PossiblyEmpty = {} // this errors. Should it?
```

https://flowtype.org/try/#0PQKgBAAgZgNg9gdzCYAoVAXAngBwKZgAKcAziQJYBGMWAogLY7ZgC8YAJLQB4CGAxhgA8Ab1RgwPAPwAuMADsArvUp4ATqgC+APnR84ckhjA5ZxMlRoMmWVmGEbUQA

Is this by design?

EDIT: inline an example
