Drupal 6: CCK computed field confusing error

I've been using Computed field CCK plugin to calculate the members's age.

The code is presented below and I'm getting confusing error ...computed_field/computed_field.module(161) : eval()'d code on line 6.

What may cause this error, what am I missing here?

if (!$node->nid) node_save($node);

$dob = $node->field_dob[0]['value'];

if(!$dob || empty($dob)) {
    $node_field[0]['value'] = 'N/A';
} else {
    $dob = $node->field_dob[0]['value'];
    $now = time();
    $then = strtotime($dob);
    $diff = date('Y', $now) - date('Y', $then);
    ...
    ...
    ...
}

Sultan

$node_field[0]['value'] = 'N/A';

change to:

$node->field[0]['value'] = 'N/A';