Is there something wrong with my code? I keep getting a "Cannot set property 'phone' of undefined" error

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

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

  info() {
    console.log(`${this.name} is a ${this.age} year old ${this.breed}; when provoked ${this.name} makes a "${this.sound}" sound.`);
  }

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

    if(hour > 8 && hour >= 20){
      return `${this.name} is currently playing around!`;
    }else{
      return `${this.name} is currently sleeping!`;
    }
  }

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

}

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('ernie','dog',1,'pug', 'Yip! Yip!');
const vera = new Pet('vera','dog',8,'border collie', 'Woof! Woof!');

ernie.owner = new Owner('Teb', '123 Main Street');
ernie.owner.phone = '(206)123-4568';

console.log(ernie.owner);