created_at field sometimes is nil

I have a strange issue on my Ruby on Rails app.
SOMETIMES when a record is being created I get this error:

A ActiveRecord::StatementInvalid occurred in conversations#create:
Mysql2::Error: Column 'created_at' cannot be null: ...
activerecord (3.2.8)
lib/active_record/connection_adapters/abstract_mysql_adapter.rb:245:in
`query'

The query generated by activerecord is this:

INSERT INTO `conversations` (`archived_by_performer`,
`archived_by_requester`, `created_at`, `deleted_by_performer`,
`deleted_by_requester`, `delta`, `locked`, `performer_hours_assigned`,
`performer_hours_assigned_at`, `performer_id`, `performer_seal_the_deal`,
`performer_seal_the_deal_at`, `request_id`, `requester_hours_assigned`,
`requester_hours_assigned_at`, `requester_id`, `requester_seal_the_deal`,
`requester_seal_the_deal_at`, `state`, `talent_id`, `unread`, `updated_at`)
VALUES (0, 0, NULL, 0, 0, 1, 0, 0, NULL, 4, 0, NULL, NULL, 0, NULL, 1, 0,
NULL, 'new', 75, 4, NULL)

It happens randomly.

Is there anything unusual about the model? Show us the code for the
action doing the create and the section of the log that corresponds to
the failing action.

Colin