user warning: Field 'timestamp' and othes doesn't have a default value query

Once I had been trying to figure out the problem by adding some log. I found:
1. in send.inc, $message['sender'] is not set in function, so the site info will be used. That's not correct as the sender info had been provided in the send form.
public function send() {
if (!$recipients = $this->recipients()) return FALSE;
if (!$message = $this->message()) return FALSE;

$modes = module_invoke_all('send_mode_info');

// Keep a running list of contacts.
$scids = array();

// Set the delivery mode based on the default and profile settings.
$mode = isset($this->mode) ? $this->mode : 'mail';

// Set the sender from the message values.
$sender = isset($message['sender']) ? $message['sender'] : (object) array(
'uid' => 1,
'mail' => variable_get('site_mail', ''),
'name' => variable_get('site_name', t('Drupal')),
);
2. in send.api.inc, there is no timestamp info in $contact before it is inserted to db.
function send_contact($mode, $contact) {
...
// Save a new entry if we didn't just load one.
if (!$contact->scid) {
drupal_write_record('send_contact', $contact);
if ($table) {
drupal_write_record($table, $contact);
}
}
...
}

But based on my Drupal knowledge I can't figure out the root cause. Wish this helpful to find out the reason. Thanks.

Regards,
Younfan

Update:
I unchecked the "not null" for the two fields "uid" and "timestamp" in send_contact table from db. Now it works.

My question:
Do these really need "not null" from the install?

See send.install:
'send_contact' => array(
'fields' => array(
'scid' => array(
'description' => t('The send contact id'),
'type' => 'serial',
'unsigned' => TRUE,
),
'uid' => array(
'description' => t('The user id'),
'type' => 'int',
'not null' => TRUE,
),
'mode' => array(
'description' => t('Delivery mode for this contact.'),
'type' => 'varchar',
'not null' => TRUE,
'length' => 32,
),
'timestamp' => array(
'description' => t('The last activity timestamp for this contact.'),
'type' => 'int',
'not null' => TRUE,
'unsigned' => TRUE,
),
),
'primary key' => array('scid'),
),

Regards,
Weijian