The add_category function of the application can add category to an email which already has a category, but can't add category to an email which does not have a category.

The issue is probably caused by this part of the function:

try:
 email = folder.get(message_id=email_id)
 email.categories.extend(category)
 email.save()

The function tries to extend the categories list, however if there is no category on the email this is a NoneType object.

Exchange version: 15.2