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 ('757b366734ae568c8c0a0040fc3a67e1', -1, 1438748326, 1438748326, '365b26dd', 19, 0, 0)

Line : 187
File : sessions.php