problem with NOTICE: _outNode: don't know how to print type

Hello,

I know (reading an older post) that this kind of message from postgres
should indicate a problem with system tables:

NOTICE:  _outNode: don't know how to print type 1862286908

but I do not know how to proceed debugging/correcting this. I get this message
from commands \l and vacuum. The SQL call which seems to cause the
problem can be demonstrated like this:

template1=> create table a (a int);
CREATE
template1=> select a from a union select a from a where a in (select a from
a);
NOTICE:  _outNode: don't know how to print type 1044119613
NOTICE:  _outNode: don't know how to print type 1044119613
a
-
(0 rows)

template1=>

The 'union' is essential for this problem to show. I have installed postgresql
7.0 on three different machines and only one of them does not give me
this notice: all Linuxes one 5.1 (not ok), one 6.2 manually updated from 5.1
(not ok) and one 6.2 new installation (ok). So it seems that the problem
may not be system tables. The type number is exactly the same in those
two machines in similar test. Besides, the psql prompt is different in these
installations but does not correlate with this problem. I ran regression tests
on one of these installations giving this problem but everything was ok
by it.

I know that probably I could get rid of this problem by re-installing redhat
6.2 on the two other machines but that would be an overkill.

Ari Jolma