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 ('604f9d216b1a813ff7f1c4c86f591601', -1, 1371725430, 1371725430, '6b159c8c', -1, 0, 0)

Line : 173
File : sessions.php