Get ID of Current User and Save it to Firebase Storage Error

Ionic Framework ionic-v3
1

af:FirebaseListObservable ;

constructor (af: AngularFireDatabase){

}

  signupUser(){
    if (!this.signupForm.valid){
      console.log(this.signupForm.value);
    } else {
      this.authData.signupUser(this.signupForm.value.email, this.signupForm.value.password)
      .then(value => {
        this.af.list('Users/').update(value.uid); // to set as Primary
        this.navCtrl.setRoot(HomePage);

      }), (error) => {
        this.loading.dismiss().then( () => {
          var errorMessage: string = error.message;
            let alert = this.alertCtrl.create({
              message: errorMessage,
              buttons: [
                {
                  text: "Ok",
                  role: 'cancel'
                }
              ]
            });
          alert.present();
        });
      };

      this.loading = this.loadingCtrl.create({
        dismissOnPageChange: true,
      });
      this.loading.present();
    }
  }

Im having an error saying that on this.af.list(‘Users/’).update(value.uid);

“Property list does not exist on type Firebaselistobservable”