[Koha] Problem in search

Dear friends i have ubuntu 12.04 and koha version 3.16.2, and when i search with some words like education or drugs the system give me this error "Can't call method "as_string" on an undefined value at /usr/share/koha/lib/C4/Koha.pm line 1608." how solve this problem it's the first time see it and i need to solve for searching and users used system. Ahmad Nasser koha system administrator future university in Egypt mobil: +201110136090 -- View this message in context: http://koha.1045719.n5.nabble.com/Problem-in-search-tp5809718.html Sent from the Koha-general mailing list archive at Nabble.com.