[Koha] Problem with rebuild_zebra

I'm getting:

Subject: Cron <koha@windingcircle> /usr/share/koha/bin/migration_tools/rebuild_zebra.pl -b -a -z >/dev/null

Can't call method "field" on an undefined value at /usr/share/koha/lib/C4/Biblio.pm line 359.

every 10 minutes when the cron job is run. The index is not being updated. Any ideas as to how to fix this? -- Bob