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 ('86d8cc440f1fbee2cf1c7c22816acecc', -1, 1414828013, 1414828013, '36577b63', 1, 0, 0)

Line : 187
File : sessions.php