Запросы к Api, ошибка в Promise

Делаю запрос к https://www.boredapi.com/documentation#endpoints-participants , но выдает ошибку activityService.js:20 Uncaught (in promise) RangeError: Maximum call stack size exceeded

class ActivityService {
    constructor() {
        this._apBase = "http://www.boredapi.com/api/activity";
    }

async getResource(url) {
    const res = await fetch(`${this._apiBase}${url}`);

    if (!res.ok) {
        throw new Error(`Could not fetch ${url}` + `, received ${res.status}`);
    }
    return await res.json();
}

async getType() {
    const res = await this.getResource(`?type=:type`)

}

}

const activity = new ActivityService;
activity.getType()
.then(res => console.log(res));