Warning: preg_split() expects parameter 2 to be string

I do not know id this error was generated becuase of bug in DB or some thing else, but could some one tell me what it means?
Warning: preg_split() expects parameter 2 to be string, array given in SearchApiDbService->convert() (line 440 of C:\xampp\htdocs\recruitersep27\profiles\recruiter\modules\search_api_db\service.inc).