Status 206 from api.ai https webhook

Question

I currently have a working ExpressJS webhook for api.ai hosted on EC2.

However, my backend fails when I use the identical GET/POST handlers in an https webhook. In particular, I don't see any (console.log) output from the POST handler, and my api.ai action returns a 206 error.

I've verified that the https webhook server is reachable from a browser. TIA for any suggestions

The api.ai JSON status is below:

{
  "id": "ff86f1da-a136-4ec2-b7f8-4aa7cb443f5a",
  "timestamp": "2017-07-16T19:04:27.522Z",
  "lang": "en",
  "result": {
    "source": "agent",
    "resolvedQuery": "add bread",
    "action": "",
    "actionIncomplete": false,
    "parameters": {
      "number": "",
      "shopping_item": "bread",
      "unit-weight-name": ""
    },
    "contexts": [],
    "metadata": {
      "intentId": "7ea8b3b1-28d6-418e-b9d2-b7353bf5a008",
      "webhookUsed": "true",
      "webhookForSlotFillingUsed": "false",
      "webhookResponseTime": 126,
      "intentName": "add shopping item"
    },
    "fulfillment": {
      "speech": "done",
      "messages": [
        {
          "type": 0,
          "speech": "okey-dokey"
        }
      ]
    },
    "score": 0.9599999785423279
  },
  "status": {
    "code": 206,
    "errorType": "partial_content",
    "errorDetails": "Webhook call failed. Error: Webhook response was empty."
  },
  "sessionId": "963f3693-5fa3-4d45-a3f3-817f9d433965"
}

The POST handler is below.

app.post('/',function(req, res){
        res.setHeader('Content-Type', 'application/json');
        //debugging output for the terminal
        console.log('you posted: Id: ' + req.body.id + ', Item: ' + req.body.result.parameters.shopping_item 
                    + ', Query: '+req.body.result.resolvedQuery);
        jsonObj={};
        jsonObj["displayText"]=req.body.result.parameters.shopping_item;
        jsonObj["textToSpeech"]=req.body.result.parameters.shopping_item;
        jsonObj["formattedText"]=req.body.result.parameters.shopping_item;
        response = "added - "+req.body.result.parameters.shopping_item;
        res.send(JSON.stringify({ "speech": response, "displayText": response}));

    });