API allows to change avatar for ServiceUser's but with no effect

When I call UserService.updateAvatar(user, base64encodedImage) with an ordinary user, the avatar gets updated as expected. However, if I call this method with a ServiceUser, I only see the default avatar image. It looks like the avatar does not get updated.

I would expect to either change the API if this is an expected behavior so that this method cannot be called with a ServiceUser or at least raise an exception or otherwise (much preferred!) to allow setting the avatar for ServiceUser's. This would be a great feature for add-ons that execute actions as a service user because they do not count to the number of users allowed in the Stash license of the customer.