x-www-form-urlencoded to JSON Azure Function

module.exports = function (context, data) {

    context.res = {
        body: parseQuery(data.form)
    };

    context.done();
};

function parseQuery(qstr) {
        var query = {};
        var a = qstr.substr(0).split('&');
        for (var i = 0; i < a.length; i++) {
            var b = a[i].split('=');
            query[decodeURIComponent(b[0])] = decodeURIComponent(b[1] || '').replace(new RegExp("\\+", 'g'), ' ');
        }
        return query;
    }

I keep getting this error:

mscorlib: TypeError: Cannot read property 'substr' of undefined
at parseQuery

Edit the function.json file and add "dataType": "stream" to the incoming binding, like this:

  "bindings": [
    {
      "authLevel": "function",
      "type": "httpTrigger",
      "direction": "in",
      "name": "req",
      "dataType": "stream"
    },