class 고양이 한살먹기 undefined가 나옵니당 8ㅅ8

class animals {
  constructor(type, color) {
    this.type = type;
    this.color = color;
  }
  nextAge() {
    if(this instanceof cats) {
      this.age++
    }
  }
}
/*
let dog1 = new animals('maltize', 'white');
let dog2 = new animals('gindotgae', 'borwn');

console.log(dog1)
 */
class cats extends animals {
  constructor(type, color, age){
    super(type, color);
    this.age = age;
  }
 
}
let cat1 = new cats('koreanshot', 'orange', 5);
let cat2 = new cats('russianblue', 'gray', 2);
console.log(cat1.nextAge())

무슨 문제인지 모르겠습니다 ㅠㅠㅠㅠㅠㅠ