XF 1.5 Field 'trigger_log_id' doesn't have a default value

Zend_Db_Statement_Mysqli_Exception: Mysqli statement execute error : Field 'trigger_log_id' doesn't have a default value - library\Zend\Db\Statement\Mysqli.php:214

Anyone know the cause of this? It happened most recently when a new user tried to register.

If it's some sort of auto-increment, what database table should I be looking at to double check if it's actually on auto-increment?