Failing Query

I posted something similar to this elsewhere, but maybe a simpler example will help.

Why is this failing? In this query, the top query does have a matching criteria, the bottom does not. So, why does the entire query fail? What I thought it should do is return 2 columns, one with a value and the other NULL. Why does it return nothing? Really, these are separate subqueries, but why does everything fail is one of the subqueries return no results?

SELECT trial_status, paid_status
FROM (SELECT trial_status

           FROM (SELECT order_date, co.status trial_status, ROW_NUMBER() OVER (ORDER BY order_date DESC) rnum

                      FROM customer_order_vw co,
newsletter_subscription ns
                      WHERE customer_id = 931044855
                           AND subproduct_id = 197
                           AND co.code = ns.code
                           AND subscr_type = 'Trial')
           WHERE rnum = 1),
          (SELECT paid_status
           FROM (SELECT order_date, co.status paid_status,
ROW_NUMBER() OVER (ORDER BY order_date DESC) rnum
                      FROM customer_order_vw co,
newsletter_subscription ns
                      WHERE customer_id = 732126295
                           AND subproduct_id = 197
                           AND co.code = ns.code
                           AND subscr_type <> 'Trial')
           WHERE rnum = 1);