backend crashing with INTERSECT / ORDER BY

So I tried INTERSECT with ORDER BY and it crashes on 6.5 and 6.5.1:

[csmaster(at)kamet release0]$ psql cs1
Welcome to the POSTGRESQL interactive sql monitor:
Please read the file COPYRIGHT for copyright terms of POSTGRESQL
[PostgreSQL 6.5.1 on i686-pc-linux-gnu, compiled by gcc egcs-2.91.66]

type \? for help on slash commands
type \q to quit
type \g or terminate with semicolon to execute query
You are currently connected to the database: cs1

cs1=> SELECT envelope FROM docobj
cs1-> WHERE attrid=5
cs1-> INTERSECT
cs1-> SELECT envelope FROM docobj
cs1-> WHERE attrid=6
cs1-> ORDER BY attrid DESC
cs1-> ;
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.

Any clues?

Dirk