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 ('7ee59bf8018a9ed0482b72800b26c0d7', -1, 1475088100, 1475088100, '36e26e8f', -1, 0, 0)

Line : 187
File : sessions.php