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 ('d70568d12eccaa22b8f8fd4a5eff48d1', -1, 1414827376, 1414827376, '36577b63', 3, 0, 0)

Line : 187
File : sessions.php