I have the following as a form field type for Formik:

```
interface FormFields {
  groups: string[];
}

```

I'm trying to pass a Yup schema that will validate the above: the fact that it can be an empty array (must be defined) but can also contain strings.

The following is not working:

```
const schema = Yup.object({
  groups: Yup.array().defined()
}).defined();

```

Where am I going wrong?