[Koha] Regardng Koha search

Hi all, I upgraded the Koha database recently from 3.6 to 3.14 and again into 3.16. Koha is showing errors (mentioned below) during the search, maybe because of the upgrade, I guess. Software error: Can't call method "as_string" on an undefined value at /usr/share/koha/lib/C4/Koha.pm line 1608. Some of the keywords I used during the search are: account, business and accounting but while other keywords don't have problem. Any idea how to resolve this problem? Thank you. -- Dibyendra Hyoju