Serverless warning: No principalId in response when there is - aws-lambda

Question

Trying to integrate serverless-local-authorizers-plugin to a system that uses AWS Lambda Authorizer. When I print the response it does contain a principalId and I return it in an object exactly like the online lambda response does. Any ideas why I am getting the error
Serverless: Warning: No principalId in response?
How it was returned:
{
"principalId":"user",
"policyDocument":{
"Version":"2012-10-17",
"Statement":[{
"Action":"execute-api:Invoke",
"Effect":"Deny",
"Resource":"arn:aws:execute-api:eu-central-1:my-AWS-ID:*"}
]},
"context":{
"platformRoles":"Guest",
"userId":"unknown"
}
}
local auth proxy function
const AWS = require('aws-sdk');
const mylocalAuthProxyFn = async (event, context) => {
const lambda = new AWS.Lambda();
const req = {
FunctionName: 'my-lambda-function-name',
InvocationType: 'RequestResponse',
Payload: JSON.stringify(event)
};
const results = await lambda.invoke(req).promise();
if (results.StatusCode === 200) {
return results.Payload;
}
throw new Error('Unauthorized');
};
module.exports = { mylocalAuthProxyFn };

Answer 1

Here is what I have figured out the works. After speaking to the library owenrs, the response has to be an explicit object in my case for some reason. They claim you can send the references variable of the promise but it didn't work for me. But the below does:
const AWS = require('aws-sdk');
const mylocalAuthProxyFn = async (event, context) => {
const lambda = new AWS.Lambda();
const req = {
FunctionName: 'aidonic-endpoints-dev-createAuthorization',
InvocationType: 'RequestResponse',
Payload: JSON.stringify(event)
};
const result = await lambda.invoke(req).promise();
if (result.StatusCode === 200) {
const pl = JSON.parse(result.Payload);
return {
principalId: pl.principalId,
policyDocument: {
Version: '2012-10-17',
Statement: [
{
Action: 'execute-api:Invoke',
Effect: 'Allow',
Resource: '*'
}
]
},
context: {
platformRoles: 'Verified Organization Representative,Registered User',
userId: '23e8320fcechi042389h02ijwqwd'
}
};
}
throw new Error('Unauthorized');
};
module.exports = { mylocalAuthProxyFn };