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 ('97395e287d91b0d9b4023246bf2c4ef9', -1, 1406252018, 1406252018, '365231c8', 0, 0, 0)

Line : 373
File : sessions.php