[REQUIRED] Describe your environment

• Operating System version: Windows 10
• Browser version: Chrome 81.0.4044.138 64 bit
• Firebase SDK version: 8.4.0
• Firebase Product: firestore

[REQUIRED] Describe the problem

When using firebase/testing to run local tests, I encounter the following problem:
when I attempt to store document containing timestamp field I get an error:
"Function DocumentReference.set() called with invalid data. Unsupported field value: a custom object (found in field encrypted_data.registrationDate)"
encrypted_data.registrationDate is the field of type Timestamp.

Steps to reproduce:

Create db connection to local simulators using '@firebase/testing'
Create timestamp value using FirebaseFirestore.Timestamp from 'firebase-admin';
Store document with timestamp value.

Same code works when using db connection from 'firebase-admin', both with local simulators and with cloud production environment.

Non-timestamp fields are stored normally.

Relevant Code:

The db connection and timestamps are obtained as follows:

import * as firebaseTest from '@firebase/testing'
import admin from 'firebase-admin';
export class ServiceDirectoryTest {
////other stuff
private async initializeFirebase() {
        this.firebaseApp = firebaseTest.initializeAdminApp({ projectId: this.projectId })
        this.prodAdminApp = admin.initializeApp()   
    }
db(): FirebaseFirestore.Firestore {
        let db :any = this.firebaseApp.firestore()
        //let db = this.prodAdminApp.firestore()
        return db;
    }

now(): FirebaseFirestore.Timestamp {
        return admin.firestore.Timestamp.now()        
    }
}