Error when trying to drop a tablespace

Hi guys,

I get the following error :

postgres=# DROP TABLESPACE IF EXISTS my_tbs;
ERROR:  tablespace "my_tbs" is not empty

I've searched in pg_class and I'm not able to find a relation which
refers to my_tbs with :

postgres=# select * from pg_class where reltablespace=100456;
 relname | relnamespace | reltype | relowner | relam | relfilenode |
reltablespace | relpages | reltuples | reltoastrelid | reltoastidxid |
relhasindex | relisshared | relkind | relnatts | relchecks | reltriggers
| relukeys | relfkeys | relrefs | relhasoids | relhaspkey | relhasrules
| relhassubclass | relfrozenxid | relacl | reloptions

---------+--------------+---------+----------+-------+-------------+---------------+----------+-----------+---------------+---------------+-------------+-------------+---------+----------+-----------+-------------+----------+----------+---------+------------+------------+-------------+----------------+--------------+--------+------------
(0 rows)

100456 has been found with :

/oid2name -s|grep my_tbs

Any idea ?
--
Cyril SCETBON