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 ('2a100c1e3b2929cff4f7f1d8af61af23', -1, 1438741171, 1438741171, '36e305c6', 3, 0, 0)

Line : 187
File : sessions.php