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 ('a7d2e2cb58b5ddb62d41b4225a1c2d32', -1, 1435682587, 1435682587, '36c66d4a', -1, 0, 0)

Line : 187
File : sessions.php