TypeError: Cannot set property 'phone' of undefined

class Pet {
  constructor(animal, age, breed, sound) {
    this.animal = animal;
    this.age = age;
    this.breed = breed;
    this.sound = sound;
  }

  get activity(){
    const today = new Date();
    const hour = today.getHours();

    if (hour > 8 && hour <= 20) {
      return 'playing';
    } else {
      return 'sleeping';
    }
  }

  set owner(owner) {
    this._owner = owner;
    console.log(`setter called: ${owner}`);
  }  

  speak() {
    console.log(this.sound);
  }

} 

class Owner {
  constructor(name, address){
    this.name = name;
    this.address = address;
  }

  set phone(phone) {
    const phoneNormalized = phone.replace(/[^0-9]/g, '');
    this._phone = phoneNormalized;
  }

  get phone() {
    return this._phone;
  }

}

const ernie = new Pet('dog', 1, 'pug', 'yip yip');
const vera = new Pet('dog', 8, 'border collie', 'woof woof');

ernie.owner = new Owner('Ashley', '123 Main Street');
ernie.owner.phone = '555-551-8971';

console.log(ernie.owner);