phpBB : Critical Error

Error creating new session

DEBUG MODE

SQL Error : 1062 Duplicate entry '-1' for key 'session_user_id'

INSERT INTO sessions (session_id, session_user_id, session_start, session_time, session_ip, session_page, session_logged_in, session_admin) VALUES ('c35bb03a3f2507e2a315a434092946e0', -1, 1425514087, 1425514087, '36a7b3d2', 33, 0, 0)

Line : 187
File : sessions.php