[Koha] check in /check out fails

Hi, Upon trying to check items out to a patron this message appears: -- Software error: Can't call method "subfield" on an undefined value at /usr/share/koha/lib/C4/Biblio.pm line 3008. The item gets checked out (i.e. it appears in the list of items checked out to the client when you go return to the checkout page). The same happens with check in. The items get checked in, but Koha stops at the error message. What can i do? best, samuel