Fails to check "Objects as maps"

My code:

```js
export type ContextTypes = {
  query: { [string]: string },
};
```

It is built fine, but when I run on a browser, the error log below appears:

```
Warning: Failed prop type: propValue.hasOwnProperty is not a function
```

Reference: [Objects as maps in Flow Types](https://flow.org/en/docs/types/objects/#toc-objects-as-maps)