Error 704 - don't know how to copy object???

Hi, it's me again...
I tried the following query which seems to be OK but returning
"error 704 -don't know how to copy object":

select kat.ident, kat,nazev, kat.popis,refs_exist.index, songs_exist.song
from kat,refs_exist,songs_exist where kat.ident='7559-26440-2' and
kat.record=refs_exist.record and songs_exist.id_song=refs_exist.id_song;

The keys are: kat.ident VARCHAR, *.record int4, *.id_song int4.

I was searching for it at the mailing list archives and the documentation but I
couldn't find the description of this error message... Could you tell me what's
wrong? It's not so complex query I hope... I just upgraded to 6.5 version.

Thanks in advance for any ideas...
--
Michal Samek, Tony distribuce s.r.o.
webmaster@tony.cz  (++420659/321350)
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