DB Error: Duplicate entry '' for key 'sessions.sessions_pkey'

Stack Trace:

File: /home/web/journal.ucyp.edu.my/lib/pkp/classes/session/SessionDAO.inc.php line 78
Function: DAO->update("INSERT INTO sessions (session_id, ip_address, user_agent, cr...", Array(8))

File: /home/web/journal.ucyp.edu.my/lib/pkp/classes/session/SessionManager.inc.php line 89
Function: SessionDAO->insertObject(Object(Session))

File: /home/web/journal.ucyp.edu.my/lib/pkp/classes/session/SessionManager.inc.php line 129
Function: SessionManager->__construct(Object(SessionDAO), Object(Request))

File: /home/web/journal.ucyp.edu.my/lib/pkp/classes/security/Validation.inc.php line 371
Function: SessionManager::getManager()

File: /home/web/journal.ucyp.edu.my/lib/pkp/classes/core/PKPPageRouter.inc.php line 66
Function: Validation::isLoggedIn()

File: /home/web/journal.ucyp.edu.my/lib/pkp/classes/core/Dispatcher.inc.php line 117
Function: PKPPageRouter->isCacheable(Object(Request))

File: /home/web/journal.ucyp.edu.my/lib/pkp/classes/core/PKPApplication.inc.php line 252
Function: Dispatcher->dispatch(Object(Request))

File: /home/web/journal.ucyp.edu.my/index.php line 4018
Function: PKPApplication->execute()