[Koha] No search results and zebra warning

Hi, Our library has Koha 3.8 installed on Debian 6 using the deb packege koha-common. I've imported a part of our UNIMARC records with items, but it seems that no record was indexed - searching for known keywords gives no results. Running koha-rebuild zebra -f {instance name} gives this warning: [warn] Record didn't contain match fields in (bib1,Local-number) Is this a problem with wrong Unimarc records in the imported batch? Any hint is appreciated! Thanks! Marius