Tengo un pequeño problema con una consulta proporcionada por Periscope. ¿Puedes ayudarme a señalarme en la dirección correcta?

El error es - ERROR: las funciones agregadas no están permitidas en GROUP BY Posición: 305

 with monthly_activity as ( select distinct date_trunc('month', created_at) as month, user_id from oauth_refresh_tokens ), first_activity as ( select user_id, date(min(created_at)) as month from oauth_refresh_tokens group by 2 ) select this_month.month, count(distinct user_id) from monthly_activity this_month left join monthly_activity last_month on this_month.user_id = last_month.user_id and this_month.month = last_month.month + interval '1 month' join first_activity on this_month.user_id = first_activity.user_id and first_activity.month != this_month.month where last_month.user_id is null group by 1