Update Field with FieldPath, update doesn't accept FieldPath as parameter - Firebase Flutter

Question

I was trying to delete a FieldValue inside Firebase, but the key had a "/" in it. So my app gave me this exception:

FirebaseException ([cloud_firestore/unknown] Use FieldPath.of() for field names containing '~*/[]'.)

After researching "FieldPath" for a while, I came up with following:

final CollectionReference userTodos = FirebaseFirestore.instance.collection('userTodos');
Future deleteToDo(String key, String listID) async {
    return await userTodos
        .doc(userID)
        .collection('Lists')
        .doc(listID)
        .update(FieldPath([key]), FieldValue.delete());
  }

But for some reason, it gives me this error:

The argument type 'FieldPath' can't be assigned to the parameter type 'Map<String, Object?>'

I don't understand why because here it says it accepts a String or FieldPath: https://firebase.google.com/docs/reference/js/v8/firebase.firestore.DocumentReference#update

Could someone help me please? Thanks :)