DevExtreme - MyApp.app.navigation[x].option("badge", "x") not working

Question

I'm trying to create a badge on a slide out navigation item, but I can't seem to make it work.

Basically I have a Login that can create Hour Registrations. These registrations needs to be either confirmed or denied by a Login with Role_ID = 1.

After a Login has logged in (if Role_ID = 1), I pull out all of these Hour Registrations from my service, and then does this:

if (data.Role.Id === 1) {
                   HourRegistrationApplication.config.navigation[7].visible(true);

                   $.getJSON(baseAddress + "hours/head/" + data.Id,
                       function (hourRegArray) {
                           if (hourRegArray.length != 0) {
                               console.log(hourRegArray.length);
                               HourRegistrationApplication.app.navigation[7].option("badge", hourRegArray.length);
                           }
                       });

               }

Where hourRegArray is the JSON that I return:

[
  {
    "ID": 1,
    "Hours": 5,
    "Date": "27/08/2015",
    "Comment": "Stackoverflow rocks",
    "Login_ID": 1
  },
  {
    "ID": 2,
    "Hours": 3,
    "Date": "25/08/2015",
    "Comment": "Stackoverflow rocks even more!",
    "Login_ID": 1
  }
]

Then I wish to have the value of badge to be the hourRegArray.length, but for some strange reason I get this error:

0x800a138f - JavaScript runtime error: Invalid operand to 'in': Object expected

What exactly is causing the problem here?

I've made a support ticket on the DevExtreme support forum, and the support told me to update my project to the latest 15.1.6 (I think it is now), didn't make any difference, I get the same error.