Why does a category idnumber return null?

I use the Moodle API and GetToken to log-in with a user. Then I retrieve the categories information here:

api.GetCategories(siteInfo.Userid, categories =>
                {
                    DataManager.Instance.categories.Clear();
                    if (categories.Length > 0)
                    {
                        foreach (var category in categories)
                        {
                            // Save category info.
                            Debug.Log(category.Idnumber);
                            DataManager.Instance.categories.Add(category);
                        }
                    }
                    queuedRequestCount--;
                });

However, category.Idnumber returns null. It shouldn't be null, it's definitely filled in.

Is it an issue with account privilege? Or what could it be..?

Thank you.