Using Aggregation Function on MongoDB

I am trying to perform this sql statement in dremio on a mongodb source. But I am getting following error message.

Cannot apply ‘SUM’ to arguments of type ‘SUM(<VARCHAR(65536)>)’. Supported form(s): ‘SUM()’.

Is there a way to perform such queries on mongdb sources in dremio? Thanks for any help.

select
l_returnflag,
l_linestatus,
sum(l_quantity) as sum_qty,
sum(l_extendedprice) as sum_base_price,
sum(l_extendedprice * (1 - l_discount)) as sum_disc_price,
sum(l_extendedprice * (1 - l_discount) * (1 + l_tax)) as sum_charge,
avg(l_quantity) as avg_qty,
avg(l_extendedprice) as avg_price,
avg(l_discount) as avg_disc,
count(*) as count_order
from
postgresql.public.lineitem
where
l_shipdate <= date ‘1998-12-01’ - interval ‘90’ day
group by
l_returnflag,
l_linestatus
order by
l_returnflag,
l_linestatus;

@vincent_mayer Are you trying to SUM of a string column? Can you first CAST to a integer type and then do the SUM?