AWS Cognitoを用いたログイン認証でUnknown Errorが出る

Question

環境

・Node.js 6.10
・AWS Lambda

現状

Node.jsを用いて, AWS Cognitoにおけるログイン認証を実現しようとしています。

'use strict';

const AWS = require("aws-sdk");
global.fetch = require("node-fetch");
const AmazonCognitoIdentity = require("amazon-cognito-identity-js");
const CognitoUserPool = AmazonCognitoIdentity.CognitoUserPool;
const common = require("./common");

exports.handler = (event, context, callback) => {
    const response = common.response();
    initilize(event, context)
        .then((data) => {
            return cognito_login(data);
        })
        .then((data) => {
            response.body = JSON.stringify(data);
            callback(null, response);
            return;
        })
        .catch((e) => {
            response.body = JSON.stringify(e);
            callback(null, response);
            return;
        });
};

// Initialize variables
const initilize = (event, context) => {
    return new Promise((resolve, reject) => {
        if (!JSON.parse(event.body) || !event.body) reject();
        else if (!JSON.parse(event.body).address) reject();
        else if (!JSON.parse(event.body).password) reject();
        else resolve(JSON.parse(event.body));
    });
};

// Login user
const cognito_login = (data) => {
    return new Promise((resolve, reject) => {
        const authenticationDetails = new AmazonCognitoIdentity.AuthenticationDetails({
            Username: data.address,
            Password: data.password
        });
        const userPool = new AmazonCognitoIdentity.CognitoUserPool({
            UserPoolId: common.get_cognito().userPoolId,
            ClientId: common.get_cognito().clientId
        });
        const cognitoUser = new AmazonCognitoIdentity.CognitoUser({
            Username: data.address,
            Pool: userPool
        });
        cognitoUser.authenticateUser(authenticationDetails, {
            onSuccess: function(result) {
                AWS.config.region = common.get_cognito().region;
                AWS.config.credentials = new AWS.CognitoIdentityCredentials({
                    IdentityPoolId: common.get_cognito().identityPoolId,
                    Logins: {
                        "cognito-idp.[region].amazonaws.com/[region]_[userpoolid]": result.getIdToken().getJwtToken()
                    }
                });
                AWS.config.credentials.refresh((err) => {
                    (err)? reject(err): resolve("Successfully login.");
                });
            },
            onFailure: function(err) {
                console.log(err);
                reject(err);
           }
        });
    });
};

上記のプログラムを実行すると、以下のようなエラーが発生します。

{
  "code": "UnknownError",
  "message": "Unknown error, the response body from fetch is: undefined"
}