Seeing this error a few times a month.

Log for the latest instance:
2019-06-05 00:00:02,558 ERROR Initialized Drupal 7.67 root directory at [notice]
2019-06-05 00:00:02,558 ERROR /srv/org.wikimedia.civicrm/drupal
2019-06-05 00:00:02,558 ERROR Initialized Drupal site default at sites/default [notice]
2019-06-05 00:00:03,605 ERROR array_flip(): Can only flip STRING and INTEGER values! [warning]
2019-06-05 00:00:03,605 ERROR Contact.php:457
2019-06-05 00:00:06,660 ERROR A fatal error was triggered [error]