not a single-group group function when trying to add two sum() function [on hold]

I have two tables:

create table a 
(
  index_ numeric(1),
  value_ numeric(2)
);
create table b 
(
  index_ numeric(1),
  value_ numeric(2)
);

Now I want to get the number of these two tables sum,I tried :

select sum(value_) - (select sum(value_) from b) from a ;

it is not working without group by, how can i do this without group by?