how to populate an array

[sarmong]

I have a collection groups, inside which there is an array students with each item being a reference to a document in another collection - students.

I am doing this

const populates = [
  { child: 'students', root: 'students' }
]
const enhance = compose(
  firestoreConnect([
    { collection: 'groups', populates }
  ]),
  connect(
    ({ firestore }) => ({
      groups: populate(firestore, 'groups', populates),
    })
  )  
)

I get this error

Uncaught FirebaseError: Function CollectionReference.doc() requires its first argument to be of type non-empty string, but it was: a custom n object