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 ('2f7c2aca69f99053a32f79cb958c3cc3', -1, 1438748338, 1438748338, '365b26dd', 14, 0, 0)

Line : 187
File : sessions.php