Computed value is not saved in database table

Not sure if this is a bug or not, but since I cannot find any other explanation I will assume it is. I am using a function in my module to compute the value of a computed field (See following function). I have confirmed that the function is being run when the node is updated, and the correct output is being generated by the function. The only problem is that the generated data ($output) is not being written to DB table.

Any ideas?

function computed_field_namecards_namecard_full_content_compute($entity, $entity_type, $entity_lang, &$entity_field, $langcode) {
//  dsm($entity_lang);
//  dsm($entity_field);
  $output = '';
  $lang = $entity_lang->language;
  $node_ref_nids = array();
  $fields = array(
    'namecards_namecard_given_name',
    'namecards_namecard_nickname',
    'namecards_namecard_organization',
    'namecards_namecard_department',
    'namecards_namecard_event',
    'namecards_namecard_position',
    'namecards_namecard_phone',
    'namecards_namecard_fax',
    'namecards_namecard_mobile',
    'namecards_namecard_email',
    'namecards_namecard_address',
  );

  // Add surname.
  $output .= check_plain($entity_lang->title) . ' ';

  // Add remaining fields.
  foreach ($fields as $field_name) {
    $count = count($entity_lang->{$field_name}[$lang]);
    for ($i = 0; $i < $count; $i++) {
      if ($field_name != 'namecards_namecard_address') {
        // Get value of all fields except address, as address
        // has a different structure and is thus handled separately.
        foreach ($entity_lang->{$field_name}[$lang][$i] as $key => $value) {
          if ($key == 'value') {
            $output .= check_plain($value) . ' ';
          }
          elseif ($key == 'nid' && preg_match('/^[0-9]{1,}$/', $value) > 0) {
            // Collect nid of referenced node.
            $node_ref_nids[] = (int) $value;
          }
        }
      }
      else {
        // Get values of address field.
        foreach ($entity_lang->namecards_namecard_address[$lang][$i] as $value) {
          $output .= check_plain($value) . ' ';
        }
      }
    }
  }

  // Get the values (i.e. titles) of all referenced nodes.
  if (count($node_ref_nids) > 0) {
    $results = db_query('SELECT title FROM {node} WHERE nid IN (:node_ref_nids)', array(':node_ref_nids' => $node_ref_nids));
    foreach ($results as $result) {
      $output .= check_plain($result->title) . ' ';
    }
  }
  dsm($output);
  // Set value of computed field.
  $entity_field[0]['value'] = $output;
}