Why does API Gateway return Internal Server Error when using a custom Lambda Authorizer?

Question

I have a listUsers function which was working perfectly with cognito authorizer.

Since I've changed it to a Lambda Authorizer, it returns a 500 HTTP status code.

serverless.yml:

provider:
    httpApi:
        cors: true
        authorizers:
          customAuthorizer:
            type: request
            functionName: custom-authorizer
functions:
    custom-authorizer:
        handler: authorizer.handler

listUsers:
  handler: src/users/index.listUsersHandler
  events:
    - httpApi:
        path: /users
        method: get
        authorizer: customAuthorizer

authorizer.js:

const { CognitoJwtVerifier } = require('aws-jwt-verify');
const Cognito = require('../shared/Cognito');

module.exports.handler = async (event) => {
    const authHeader = event.headers.authorization;
    if (!authHeader) {
        console.log('No auth header');
        return {
            isAuthorized: false,
        };
    }
    const token = authHeader.split(' ')[1];
    console.log(token);
    const verifier = CognitoJwtVerifier.create({
        userPoolId: Cognito.UserPoolId,
        tokenUse: 'access',
        clientId: Cognito.ClientId,
    });
    let payload = null;
    try {
        payload = await verifier.verify(token);
        console.log('Token is valid. Payload:', payload);
        return {
            isAuthorized: true,
        };
    } catch {
        console.log('Token is invalid.');
        return { isAuthorized: false };
    }
};

When I make a GET request to /users it correctly redirects to my custom-authorizer & in the logs I can see that the token is valid.

However, it never runs the actual listUsers function and in Postman I get:

{
    "message": "Internal Server Error"
}

Answer 1

I fixed this by adding this:

authorizers:
      customAuhtorizer:
         enableSimpleResponses: true