Error creating index (v6.5)

I'm trying to create the third index in a table populated with 23000 rows
and I get :

sanluis=> CREATE INDEX art_idx2 ON ARTICULOINFO (shFamilia);
FATAL 1: btree: failed to add item to the page in _bt_sort (2)
pqReadData() -- backend closed the channel unexpectedly.
This probably means the backend terminated abnormally
before or while processing the request.
We have lost the connection to the backend, so further
processing is impossible. Terminating.

I can add more index on the same table without problem using other columns,
but once the error it seems that art_idx2 doesn't exist but I can recreate
it, obtaining :

sanluis=> CREATE INDEX art_idx2 ON ARTICULOINFO (shFamilia);
ERROR: cannot create art_idx2

I've got postgres 6.5 under Linux 2.2.5
What happens?

Thanks in advance.

Roberto

What happens?