PDOException: SQLSTATE[HY000]: General error: 1366 Incorrect integer value

Hi,

Drupal's watchdog records a lot of these while people are editing opening hours:

PDOException: SQLSTATE[HY000]: General error: 1366 Incorrect integer value:
'' for column 'category_tid' at row 1:
     UPDATE {opening_hours}
     SET start_time = :start, end_time = :end, category_tid = :tid, notice =
:notice
     WHERE customised = 0 AND original_instance_id = :id
   ; Array
(
     [:start] => 09:00
     [:end] => 19:00
     [:tid] =>
     [:notice] =>
     [:id] => 1176
)
  in _opening_hours_instance_update() (Zeile 242 von
/var/web/drupal/d7contrib/opening_hours/includes/opening_hours.pages.inc).

We decided not to use terms for the opening hours. I think it's because "tid" is missing a value. Should it be NULL?