finding a max value

This is the query:
select max(KA) from annuncio

field KA is indexed and is int4,

explaining gives:
explain select max(KA) from annuncio;
QUERY PLAN
-----------------------------------------------------------------------
Aggregate (cost=21173.70..21173.70 rows=1 width=4)
-> Seq Scan on annuncio (cost=0.00..20326.76 rows=338776 width=4)
(2 rows)

wasn't supposed to do an index scan? it takes about 1sec to get the result.