Sequential scan from simple query

Can anybody tell me why the following query requires a full table scan?

pgslekt=> explain select person_id, last_edit from persons
               order by last_edit desc, person_id desc limit 50;
                               QUERY PLAN
-------------------------------------------------------------------------
 Limit  (cost=1495.03..1495.15 rows=50 width=8)
   ->  Sort  (cost=1495.03..1535.54 rows=16204 width=8)
         Sort Key: last_edit, person_id
         ->  Seq Scan on persons  (cost=0.00..362.04 rows=16204 width=8)
(4 rows)

'persons' is a regular table where person_id is the primary
key. 'last_edit' is a regular date field. I've also tried:

pgslekt=> create index last_edit_key on persons(last_edit);

But that doesn't make any difference.
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