Function DocumentReference.set() called with invalid data. Unsupported field value: a custom object (found in field z)

I’ve been trying to make register functions and store them in database in firebase even tho the users get created and they can successfully login and i see them in database i still get this error, makes no sense to me getting the error but still the users are getting created.

This is the typescript code

import { Component, OnInit } from '@angular/core';
import { Router } from "@angular/router";
import { AuthenticationService } from "../shared/authentication-service";
import {AlertController} from '@ionic/angular'
import {UserService} from '../user.service'
import {AngularFirestore} from '@angular/fire/firestore'


@Component({
  selector: 'app-register',
  templateUrl: './register.page.html',
  styleUrls: ['./register.page.scss'],
})

export class RegisterPage implements OnInit {

  constructor(
    public authService: AuthenticationService,
    public router: Router,
    public alert: AlertController,
    public user: UserService,
    public afstore: AngularFirestore,
    public alertController: AlertController
  ) { }

  ngOnInit(){}

  async presentAlert(title:string, content:string){

    const alert = await this.alertController.create({
      header: title,
      message: content,
      buttons: ["OK"]
    })

    await alert.present()
  }

  signUp(username, password){
    this.authService.RegisterUser(username.value +'@gmail.com', password.value)
    .then((res) => {

     this.afstore.doc(`users/${res.user.uid}`).set({
        username 
      })

      this.user.setUser({
        username,
        uid: res.user.uid
      })

      this.presentAlert('Success', 'Register Complete')
      this.router.navigate(['login']);
    }).catch((error) => {
     this.showAlert("Error", error.message)
    })
}

async showAlert(header:string, message:string){
  const alert = this.alert.create({
    header,
    message,
    buttons:["Ok"]
  })
  await (await alert).present()

}

}

This is the error

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