return null

SELECT gman_servers_set('127.0.0.1:4730');
+------------------------------------+
| gman_servers_set('127.0.0.1:4730') |
+------------------------------------+
| 127.0.0.1:4730 |
+------------------------------------+
1 row in set (0.00 sec)

select gman_do_background('test','test');
+-----------------------------------+
| gman_do_background('test','test') |
+-----------------------------------+
| NULL |
+-----------------------------------+
1 row in set (0.00 sec)

why return null ?