AWS Lambda error: HTTP 502 ‘Execution failed due to configuration error: Malformed Lambda proxy response’

Calling an AWS Lambda via API Gateway with the Lambda Proxy Integration option, you might see an HTTP 502 response and this message:

“Execution failed due to configuration error: Malformed Lambda proxy response"

Wed May 30 05:00:41 UTC 2018 : Execution failed due to configuration error: Malformed Lambda proxy response
Wed May 30 05:00:41 UTC 2018 : Method completed with status: 502

This is a rather cryptic message, but  it’s saying is the response is not in the expected format.

Per this doc, the expected response should be in this format:

{
    "isBase64Encoded": true|false,
    "statusCode": httpStatusCode,
    "headers": { "headerName": "headerValue", ... },
    "body": "..."
}