Flow complaining about incompatible type in object default values destructuring

I have this very simple function I'm trying to validate with Flow:

// @flow
type Props = {
  width: string | number,
};

function fun({
  width = '30em',
}: Props) {
  return width;
}

The problem is that I get this error:

8:   width = '30em',
     ^ number. This type is incompatible with
8:   width = '30em',
     ^ string
8:   width = '30em',
     ^ string. This type is incompatible with
8:   width = '30em',
     ^ number
8:   width = '30em',
             ^ string. This type is incompatible with
8:   width = '30em',
     ^ number

I wonder what I'm doing wrong... This other way works fine:

// @flow
type Props = {
  width: string | number,
};

function fun(props: Props) {
  const {
    width = '30em',
  } = props;
  return width;
}

And this syntax inside function arguments seem supported because:

// @flow
type Props = {
  width: string,
};

function fun({ width = '30em' }: Props) {
  return width;
}

This works just fine.

Ideas?