Reference.push failed: first argument path specified exceeds the maximum depth

I’m trying to send form data to firebase and route user based on selection. The routing based on selection works , but I get this error when trying to push data

component.ts

export class HomeComponent {
  // [x: string]: any;
  home:{ name: string ,
  age: number,
  mobileNo: number,
  area: string,
  address: string,
  test?: string } = { 
    name:"",
    age: null,
    mobileNo:null,
    area: "",
    address:"",
    test: null, 
  }
  testMethod = [
    { id:1, name: 'YES' , value:'positive'},
    { id:2, name: 'NO' ,  value:'negative'},
  ]

  constructor(private homeService: HomeService ,private router: Router ,  
    public authService: AuthService, 
    public ngZone: NgZone,
    private db: AngularFireDatabase) { 
  }
  
save(userDetails){
  JSON.parse(JSON.stringify(this.home))
  console.log(this.home)
  this.homeService.create(userDetails)
  // Routing based on user data
  if(this.home.test === 'positive'){
    this.router.navigateByUrl('positive'); 
  }else{
    this.router.navigateByUrl('negative');
  }
}
}

Service.ts

   constructor(  private db: AngularFireDatabase ) {    
       }
       create(userDetails){
        return this.db.list('/userDetails').push(userDetails);
      }
     }[errorwhen trying to push data ][1]

Source: Angular Questions