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 ('4b1486382eba5f6cb4341ab19ed12a87', -1, 1414818541, 1414818541, '3213a925', -1, 0, 0)

Line : 187
File : sessions.php