AWS lambda api gateway error “Malformed Lambda proxy response”

Question

My AWS ApiGateway invokes this lambda function :

const handler = (event, context) => {

    const theResponse = {
        "statusCode": 200,
        "isBase64Encoded": false,
        "headers": {
            "Content-Type": "application/json"
        },
        "body": "hello, world"
    };

    switch (event.httpMethod) {
        case 'GET':
            theResponse["body"] = `method ${event.httpMethod} detected`;
            break;
        default:
            theResponse["statusCode"] = 404;
            theResponse["body"] = `Unsupported method ${event.httpMethod} detected`;
            break;
    }
    return theResponse;
};


module.exports = {
    handler,
};

Any thoughts to why this errors with :

{"message": "Internal server error"}
Received response. Status: 200, Integration latency: 344 ms
Endpoint response body before transformations: null
Execution failed due to configuration error: Malformed Lambda proxy response
Method completed with status: 502

I tried replacing return (theResponse); with return JSON.stringify(theResponse); but this also returns the same error.

However if I replace return theResponse; with return {"statusCode":200, "body":"hello, world"}; then the API executes without errors.

Looks to me like a lamda_proxy integration issue but I can't see why. thanks.

Guina Costa · Accepted Answer · 2020-12-29 20:22:59Z

1

Bearrider!

I guess it is related to the way you are building your response object:

Try stringify only the body attribute:

    case 'GET':
        theResponse["body"] = JSON.stringify(`method ${event.httpMethod} detected`);
        break;
    default:
        theResponse["statusCode"] = 404;
        theResponse["body"] = JSON.stringify(`Unsupported method ${event.httpMethod} detected`);
        break;
    }

answered Dec 29, 2020 at 20:22

Guina Costa

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1 Comment

bearrider Over a year ago

JSON.stringify did not work but changing module.exports = { handler, }; to exports.handler worked. But I am not sure why. I thought the two are syntactically equal?

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