RawSQL window functions break count()

[Jamie Cockburn]

I wanted to annotate my objects with a running total:

class A(models.Model):
    amount = models.IntegerField()
    created = models.DateTimeField(auto_now_add=True)

qs = A.objects.annotate(total=RawSQL("SUM(amount) OVER (ORDER BY created)", []))

That works fine, and I get a running total for each object, but I cannot call count() on that queryset:

>>> qs.count()
Traceback...
ProgrammingError: window functions not allowed in GROUP BY clause

Using extra(), I can get the same annotation behaviour as well being able to call count():

>>> qs = A.objects.extra(select={'total': 'SUM(amount) OVER (ORDER BY created)'})
>>> qs.count()
8