INSERT ... SELECT UNION with parenthesis

MariaDB [test]> INSERT INTO t3 (SELECT a, NULL AS b FROM t2);

Query OK, 3 rows affected (0.08 sec)

Records: 3  Duplicates: 0  Warnings: 0

MariaDB [test]> INSERT INTO t3 (SELECT a, NULL AS b FROM t1 UNION SELECT * FROM t2);

ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'UNION SELECT * FROM t2)' at line 1

MariaDB [test]> INSERT INTO t3 SELECT a, NULL AS b FROM t1 UNION SELECT * FROM t2;

Query OK, 6 rows affected (0.08 sec)

Records: 6  Duplicates: 0  Warnings: 0