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 ('8bff83d10556e5c57c031c35c8e95381', -1, 1414681663, 1414681663, '36524d32', -1, 0, 0)

Line : 187
File : sessions.php