Project 3 Case study 2 task B syntax check

Hi I am Divakar. I was trying to do the 3rd project where in Case Study 2 task B but after many attempts I am not able to get the correct syntax.Every time I am getting the error as Error Code: 1525. Incorrect DATETIME value: '' Please check my syntax and please tell me where I am going wrong select monthyear, nousers, growthinperc, round(((activeusers / lag(activeusers, 1) over(order by monthyear)-1)*100),2) as growthin_perc from ( select concat(month(createdat),"-",year(createdat)) as month_year, count(distinct activatedat) as nousers from users where activated_at not in("") )sub;