A few weeks ago we encountered an issue with monetdb when summing nulls, grouped by some column. We managed to create a simple test case which reproduces this scenario:

create table test (number int, name varchar(50));

insert into test values(1,'jonh');
insert into test values(2,'mike');
insert into test values(3,'mary');
insert into test values(null,'austin');
insert into test values(null,'jonh');
insert into test values(null,'mary');

select name, sum(number)
from test
group by name;

austin | 0
jonh | 1
mary | 3
mike | 2

austin | null
jonh | 1
mary | 3
mike | 2