UNION ALL on Aggregates with GROUP BY returns null when column aliases don't match

select x
from
(SELECT Sum(ss_ext_sales_price) x
FROM  store_sales
UNION ALL
SELECT Sum(cs_ext_sales_price) y
FROM catalog_sales) tmp
GROUP BY x;

Results from Drill:
+------------+
|     x      |
+------------+
| null       |
+------------+
1 row selected (3.458 seconds)

Results from Postgres:
       x       
---------------
 3658019159.35
 5265207074.51
(2 rows)

select x
from
(SELECT Sum(ss_ext_sales_price) x
FROM  store_sales
UNION ALL
SELECT Sum(cs_ext_sales_price) y
FROM catalog_sales) tmp

Results from Drill:
+------------+
|     x      |
+------------+
| 5265207074.51 |
| 3658019159.35 |
+------------+
2 rows selected (3.46 seconds)