TypeError: Cannot set property 'gemPrefab' of undefined

Creator 2.x
#1

const {ccclass, property} = cc._decorator;

@ccclass
export default class Game extends cc.Component {

gemPrefab: cc.Node = null;

start() {
    cc.loader.loadRes("prefab/Gem", this.resLoaded);
}

resLoaded(err: Error, prefab: cc.Node) {
    //this.gemPrefab = prefab;
    let gem = cc.instantiate(prefab);
    let root = cc.find("Canvas/GemRoot");
    gem.setParent(root);
}

}

求助，我知道这是小白问题，但就是坑了，这段代码编译成功，预制件实例化到屏幕上了，但是一旦把this.gemPrefab = prefab;这行注释取消就报错：TypeError: Cannot set property ‘gemPrefab’ of undefined，这是为什么啊我只是给gemPrefab赋值，为啥会报这个错，prefab绝对有值，注释掉这行是可以实例化成功的