Set enable_seqscan doesn't take effect?

postgres=# SET enable_seqscan = off;
SET
postgres=# explain select * from t;
                               QUERY PLAN                                
-------------------------------------------------------------------------
 Seq Scan on t  (cost=10000000000.00..10000000023.60 rows=1360 width=32)
(1 row)

postgres=#  select * from t;
   a   
-------
 [1,2]
(1 row)