phpBB : Critical Error

Error creating new session

DEBUG MODE

SQL Error : 1062 Duplicate entry '-1' for key 'session_user_id'

INSERT INTO phpbb_sessions (session_id, session_user_id, session_start, session_time, session_ip, session_page, session_logged_in, session_admin) VALUES ('515b57a6128ff69b3ec992e28a738cb2', -1, 1414016325, 1414016325, '36cd6223', 5, 0, 0)

Line : 187
File : sessions.php