this.app.service(…).create(…) is not a function

[hhchow7]

I tried to make api request twice and the first one work, but the second fails with below error.

{
    "name": "GeneralError",
    "message": "this.app.service(...).create(...) is not a function",
    "code": 500,
    "className": "general-error",
    "data": {},
    "errors": {}
}

result.class.js

/* eslint-disable no-unused-vars */
const errors = require('@feathersjs/errors');

exports.Result = class Result {
  constructor (options, app) {
    this.app = app;
  }

  async create (data, params) {
    if (Array.isArray(data)) {
      return Promise.all(data.map(current => this.create(current, params)));
    }

    let result = await this.app.service('result').create({ //<------------- It works
      sectionId: data.sectionId,
    })
    console.log(result) // <------------------------------------ able to show the value
    (data.sub_sections).map(async sub_section =>{
      (sub_section.questions).map(async question =>{
        let payload = {
          answerId: question.questionId
        }
        await this.app.service('result').create(payload) //<------------- It results in error 
      })
    })
    return data;
  }

};