Firestore Error: [cloud_firestore/unknown] Invalid argument (dartObject): Could not convert: Instance of '_FieldValueServerTimestamp'

I'm using flutter and firestore and what I want to do is adding an object as a child inside a list, each time I hit a submit button. Inside that object, I want to have a createdAt field, where the timestamp information should be stored. For that, i'm trying to use FieldValue.serverTimestamp(), but i'm getting the following error.

Uncaught (in promise) Error: [cloud_firestore/unknown] Invalid argument (dartObject): Could not convert: Instance of '_FieldValueServerTimestamp'

The code is as below:

reference.update({
  'parentData': FieldValue.arrayUnion([
    {
      'data': myData,
      'createdAt': FieldValue.serverTimestamp()
    }
  ]),
});

I'm also importing the library:

import 'package:cloud_firestore/cloud_firestore.dart';