Typo in avg_child_age example?

[mnieber]

Hi, this library looks very nice.

I have a question about the last example:

Parent.objects.annotate(avg_child_age=SubqueryArrayAgg('child__age'))

Is there a typo here? I don't understand how this would compute the average. If there is a typo or mistake, I'd be interested to know the correct solution.