8.3.5: Types with typnamespace pointing at non-existent pg_namespace oid

A medium-length story short, this query returns non-zero:

select count(distinct typnamespace) from pg_type where not exists
(select 1 from pg_namespace where oid = pg_type.typnamespace);

I did a very brief search in all the release notes for 8.3.5 to
8.3.14, but have not found precisely what I was looking for (searches
for namespace, schema, type, and corruption).

This was discovered when performing a pg_dump of this user's database,
whereby pg_dump complained when trying to dump types for lack of a
good catalog entry to nab the namespace name from. In our case, two
namespaces seem to be affected. The user of this database was never
privileged enough to even perform CREATE SCHEMA, to my knowledge, and
in fact only have the schema (owned by the postgres superuser) that
they began with.

Is it safe to perform an UPDATE on pg_type to give entries a valid
typnamespace? Is there any forensic evidence I can grab before doing
that to assist in figuring out the mechanism for this bug, if
applicable?

Cheers.

--
fdr