Updating with NULL values + numbers

[Yarick Glodov]

Good day,

Here is a query:
update rents as r set r.Debt = r.Deposit - (select sum(e.Total) from rents_extend as e where e.RentId = r.Id) + (select sum(f.Fee) from rents_fee as f where f.RentId = r.Id);

Not in all times in rents_extend or rents_fee we have rows, so usually one of these values is NULL.
I cannot use isnull() function here, because it is subquery.
How can I deal with it?