Backend closes caused by query on a view?!

Hi!

Can anybody tell me, what I'm doing so wrong,
that the backend needs to close the connection?
I've got a view that gives me a total of costs for
each day, which looks like this:

create view view_test AS
select date(datum_zeit),sum(betrag) AS "summe"
from anrufe_isdn where richtung='True'
AND date(datum_zeit)>date(timemi(now() ::datetime, '30 day' ::timespan))
group by date(datum_zeit);

It works very well, if I do the query:

select * from view_anrufe;
or
select date,summe from view_anrufe;

But If I do:

select summe from view_anrufe;
or
select avg(summe) from view_anrufe;

I get:

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'm running Postgresql Version 6.4 on a Linux-box with Kernel V.2.0.33....

Thanks for any help in advanced!!

Viktor