Push not working here, can anyone help?

Ionic Framework
1

I don’t know what seems to be the problem here:

export class ProfilePage implements OnInit {

  data: Observable<any[]>;
  ref: AngularFireList<any>;

async addTransaction() {
    this.ref.push(this.transaction).then(() => {
      this.transaction = {
        value: 0,
        expense: false,
        month: 0,

and the error

type or paste code hereERROR Error: Uncaught (in promise): TypeError: Cannot read property 'push' of undefined
TypeError: Cannot read property 'push' of undefined