POST Request body Is Null or Empty

I am trying to make a POST request to a knowledgebase. I can get the correct response from the request maybe 5-10% of the time. All the other times I get the error in the title back from the server:

No argument passed{“Error”:{“Code”:“BadArgument”,“Message”:“Request body Is Null or Empty.”}}  

I have a feeling this is caused by Node.js being async and my variable is still undefined when the request goes through. Although, how can it when req.write() includes the variable? Maybe I can insert a delay to insure the variable is defined before the request is sent?

var https = require('https');

var resData = "";

var options = {
    host: "westus.api.cognitive.microsoft.com",
    port: 443,
    path: "/qnamaker/v2.0/knowledgebases/<kb-key>/generateAnswer",
    method : 'POST',
    headers: {
        'Content-Type': 'application/json',
        "Ocp-Apim-Subscription-Key":"<sub-key>",
    },
};

bot.dialog('qnaReq', function (session, args) {
    //call QnA Bot and ask that bot the question
var req = https.request(options, function(res) {

    res.on('data', function (chunk) {
        resData += chunk;
    });

    res.on('error', function(e) {
    console.log('problem with request: ' + e.message);
    });

    res.on('end', function() {
        if (resData.length != 75) { //75 is the length of the error I get almost every time. This line prevents the application from crashing since I am trying to access values that won't be there.
        var accessibleData = JSON.parse(resData);
        session.send(accessibleData["answers"][0]["answer"]);
        } else {
            session.send("No argument passed" + resData);
        }
        resData = "";
    });
});

    var postData = {question: session.message.text};
    console.log(postData); //postData is defined

    req.write(JSON.stringify(postData));
    req.end();

}).triggerAction({
    matches: 'IT Help'
});