enable_seqscan to off -> initial cost 10000000000

Hi all,
this could be trivial, but I would like an explaination: if I turn off
sequential scans on a table without indexes, the same access plan is
increased by a 10000000000 factor. I suspect this is a warning for me
to remind that something is misconfigured, or is there anothe reason?

testdb=# create table tseq( pk serial );
CREATE TABLE
testdb=# explain select * from tseq;
                      QUERY PLAN
--------------------------------------------------------
Seq Scan on tseq  (cost=0.00..35.50 rows=2550 width=4)
(1 row)

testdb=# set enable_seqscan to off;
SET
testdb=# explain select * from tseq;
                               QUERY PLAN
---------------------------------------------------------------------------
Seq Scan on tseq  (cost=10000000000.00..10000000035.50 rows=2550 width=4)
(1 row)

testdb=# show server_version;
server_version
----------------
12.5

Thanks,
Luca