Cognito Trigger Lambda не может подключиться к Appsync, отвечает {"size": 0, "timeout": 0}

Question

У меня есть лямбда, целью которой является получение событий Cognito Post Confirmation и использование некоторых из этих данных события для вызова мутации createUser через AppSync. Лямбда получает следующий ответ от AppSync: {"size":0,"timeout":0}. Я не могу найти документы о том, что это значит, и мутация не происходит; Кроме того, та же самая мутация и те же учетные данные прекрасно работают из консоли AppSync. Я что-то упустил очевидное?

Lambda

const URL = require("url");
const fetch = require("node-fetch");
const { CognitoIdentityServiceProvider } = require("aws-sdk");

const cognitoIdentityServiceProvider = new CognitoIdentityServiceProvider({
  apiVersion: "2016-04-18"
});
const initiateAuth = ({ clientId, userPoolId, username, password }) =>
  cognitoIdentityServiceProvider
    .adminInitiateAuth({
      AuthFlow: "ADMIN_NO_SRP_AUTH",
      AuthParameters: {
        USERNAME: username,
        PASSWORD: password
      },
      ClientId: clientId,
      UserPoolId: userPoolId
    })
    .promise();

exports.handler = async (event, context, callback) => {
  console.log(event);
  const clientId = process.env.COGNITO_CLIENT_ID;
  const userPoolId = process.env.COGNITO_USER_POOL_ID;
  const endPoint = process.env.APPSYNC_GRAPHQL_ENDPOINT;
  const username = process.env.COGNITO_USERNAME;
  const password = process.env.COGNITO_PASSWORD;
  const { AuthenticationResult } = await initiateAuth({
    clientId,
    userPoolId,
    username,
    password
  });
  const accessToken = AuthenticationResult && AuthenticationResult.AccessToken;
  console.log(`Access Token: ${accessToken}`);
  const postBody = {
    query: `mutation CreateUser($id: ID!, $username: String!) {
      createUser(input: {id: $id, username: $username}) {
        id,
        username
      }
    }`,
    operationName: "CreateUser",
    variables: {
      id: event.request.userAttributes.sub,
      username: event.username
    }
  };

  const uri = await URL.parse(endPoint);
  console.log(uri);

  const options = {
    method: "POST",
    body: JSON.stringify(postBody),
    headers: {
      host: uri.host,
      "Content-Type": "application/json",
      Authorization: accessToken
    }
  };
  const response = await fetch(uri.href, options);
  console.log(`AppSync mutation response: ${JSON.stringify(response)}`);
  const { data } = await response.json();

  const result = data && data.createUser;
  callback(null, result);
};

Шаблон SAM

AWSTemplateFormatVersion: '2010-09-09'
Transform: AWS::Serverless-2016-10-31
Description: Stack for using Cognito events to create Users database. stack-ingest-cognito-events
Resources:
  IngestCognitoEventsLambdaRole:
    Type: AWS::IAM::Role
    Properties:
      RoleName: role-ingest-cognito-events-lambda
      AssumeRolePolicyDocument:
        Version: '2012-10-17'
        Statement:
        - Effect: Allow
          Principal:
            Service: lambda.amazonaws.com
          Action: sts:AssumeRole
      Policies:
      - PolicyName: policy-ingest-cognito-events-lambda
        PolicyDocument:
          Version: '2012-10-17'
          Statement:
          - Effect: Allow
            Action:
            - logs:CreateLogGroup
            - logs:CreateLogStream
            - logs:PutLogEvents
            Resource: arn:aws:logs:*:*:*
          - Effect: Allow
            Action:
            - cognito-idp:Admin*
            Resource:
              Fn::Sub: arn:aws:cognito-idp:${AWS::Region}:${AWS::AccountId}:userpool/us-east-1_mypool
      ManagedPolicyArns:
      - arn:aws:iam::aws:policy/service-role/AWSLambdaBasicExecutionRole
  IngestCognitoEventsLambda:
    Type: AWS::Serverless::Function
    Properties:
      FunctionName: lambda-ingest-cognito-events
      Description: Ingests Cognito events, propogates changes.
      AutoPublishAlias: live
      Runtime: nodejs8.10
      Handler: index.handler
      CodeUri: s3://mybucket
      MemorySize: 128
      Timeout: 10
      Environment:
        Variables:
          COGNITO_CLIENT_ID: myclientid
          COGNITO_USER_POOL_ID: us-east-1_mypool
          APPSYNC_GRAPHQL_ENDPOINT: https://myhash.appsync-api.us-east-1.amazonaws.com/graphql
          COGNITO_USERNAME: serviceAcctUsername
          COGNITO_PASSWORD: serviceAcctPassword
      Role:
        Fn::GetAtt:
        - IngestCognitoEventsLambdaRole
        - Arn