How annotate the Max value of two fields in a Django QuerySet

I have a model Client, how do i annotate then sort, the Max of its two fields:

from django.db import models

class Client(models.Model):
    uploaded_photo_at = models.DateTimeField()
    uploaded_document_at = models.DateTimeField()

The following:

Client.objects.annotate(
    latest_activity_at=Max('uploaded_photo_at', 'uploaded_document_at', output_field=DateTimeField())
).order_by('latest_activity_at')

Raises this error:

django.db.utils.ProgrammingError: function max(timestamp with time zone, timestamp with time zone) does not exist
LINE 1: ...oto_at", "clients_client"."uploaded_document_at", MAX("clien...
                                                             ^
HINT:  No function matches the given name and argument types. You might need to add explicit type casts.

I am using Posgresql and Django 1.11, if that helps.