Not getting proper answer

In Project 3 Case Study 2, Quetstion number 2, User Growth. I'm using this query "select year, weeknum, numusers, sum(num_users) over (order by year, weeknum) as cumusers from ( select extract(year from createdat) as year, extract(week from createdat) as weeknum, count(distinct userid) as num_users from users where state='active' group by year,week_num order by year, week_num)sub;" and in return im getting zero rows, i dont understand is there any issues with data or i'm using wrong query for these. date format in user table is yyyy-mm-dd hh:mm:ss.