Pass a parameter to select case with group by in HQL

Question

I have the following HQL(The Hibernate Query Language) query (which is executed on oracle db connection) to retrieve an aggregated sum, based on month

select sum(doc._masskg), 
    case when (month(doc._date) = month(:m)) then 'NOW' else 'BEFORE' 
from Document doc 
    where month(doc._date) <= month(:m) 
group by 
    case when (month(doc._date) = month(:m)) then 'NOW' else 'BEFORE' end

however, in the runtime I get Oracle exception NOT A GROUP BY EXPRESSION

if I replace :m with sysdate, the query works. How do I rewrite the query if m: is Date?