serial field does not work in user table

i just added the boxnr as serial field to user settings area..

[ http://localhost/drupal712/#overlay=admin/config/people/accounts/fields/... ]

then i tried to new user registration and it doesnt work,
i was expecting to assign unique boxnr to users.. it gives error message

Error
The website encountered an unexpected error. Please try again later.

Error message
•Notice: Undefined property: stdClass::$nid in serial_field_insert() (line 83 of C:\xampp\htdocs\drupal712\sites\all\modules\serial\serial.module).
•PDOException: SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'nid' cannot be null: INSERT INTO {serial_user_field_boxnr} (nid) VALUES (:db_insert_placeholder_0); Array ( [:db_insert_placeholder_0] => ) in _serial_generate_value() (line 155 of C:\xampp\htdocs\drupal712\sites\all\modules\serial\serial.inc).