DB Error: Duplicate entry 'NULL' for key 'sessions_user_id'

Stack Trace:

File: /opt/html/lib/pkp/classes/session/SessionDAO.inc.php line 86
Function: DAO->update("INSERT INTO sessions (session_id, ip_address, user_agent, cr...", Array(7))

File: /opt/html/lib/pkp/classes/session/SessionManager.inc.php line 81
Function: SessionDAO->insertSession(Object(Session))

File: /opt/html/lib/pkp/classes/session/SessionManager.inc.php line 120
Function: SessionManager->SessionManager(Object(SessionDAO), Object(Request))

File: /opt/html/classes/security/Validation.inc.php line 281
Function: SessionManager->getManager()

File: /opt/html/lib/pkp/classes/core/PKPPageRouter.inc.php line 66
Function: Validation->isLoggedIn()

File: /opt/html/lib/pkp/classes/core/Dispatcher.inc.php line 117
Function: PKPPageRouter->isCacheable(Object(Request))

File: /opt/html/lib/pkp/classes/core/PKPApplication.inc.php line 190
Function: Dispatcher->dispatch(Object(Request))

File: /opt/html/index.php line 66
Function: PKPApplication->execute()