Bug with UNION and constant columns

mysql> ( SELECT '' ) UNION ( SELECT 'aaaaaaaaaaaa' );
+--+
|  |
+--+
|  |
+--+
1 row in set (0.00 sec)

mysql> ( SELECT '******' ) UNION ( SELECT '|||||||||||');
+--------+
| ****** |
+--------+
| ****** |
| |||||| |
+--------+
2 rows in set (0.00 sec)

(there are more '|' then '*')

Bug exists in 4.0.20.

Does not occur in 4.1.1.