[Koha] Koha search error

Hi all, Just recently after upgrading the Koha from 3.14 to 3.16 we have been facing following problem when certain keywords are used during the search like: account, business, accounting, financial, financial accounting, etc. Earlier there were no such problems. But when I search other keywords apart from the mentioned keywords, there are no problems. Has anyone faced this kind of problem and found a solutions already? Any help would be appreciated. Thanks. Software error: Can't call method "as_string" on an undefined value at /usr/share/koha/lib/C4/Koha.pm line 1608. Regards, Dibyendra Hyoju