SQL & PL/SQL

I have the following select to extract a date containing month and year from a varchar2 column

select to_char(sau_date,'MM') MONTH,extract(year from sau_date) YEAR, count(*) COUNT
from mac.sysaud
group by to_char(sau_date,'MM'),extract(year from sau_date)
order by extract(year from sau_date), to_char(sau_date,'MM');

when running select i receive a ora-30076:invalid extract field for extract source