Weirdness with OIDs and JOIN ON?

Why doesn't this work:

test=# select oid, relname, indisclustered from pg_index join pg_class
on indexrelid=oid where indexrelid > 17205;
ERROR: column "oid" does not exist

I'm _joining_ on the oid column.

If I qualify it, it works:

test=# select pg_class.oid, relname, indisclustered from pg_index join
pg_class on indexrelid=oid where indexrelid > 17205;
oid | relname | indisclustered
-------+--------------+----------------
17214 | child_b_key | t
17210 | parent_a_key | t
(2 rows)

I can't see that I've made an error in the first example - is it a
Postgres bug?

Chris