Need table alias in query when using idlist option with migrate-import drush command

I have a use case where during the migration of a taxonomy vocabulary from a D6 site to a D7 site, I need to split out the terms into two separate, new, vocabularies. Doing the splitting is easy enough, and I can do it with a custom table, an extension of DrupalTerm6Migration, and an implementation of prepareRow().

function nb_migrate_register_migrations() {
  // Define arguments used for all migrations.
  $common_arguments = array(
    'source_connection' => 'legacy',
    'source_version' => 6,
  );

  $vocabulary_arguments = array(
    array(
      'class_name' => 'NBPeopleOrganizationsTermMigration',
      'description' => t('Migration of People and Organizations terms from Drupal 6'),
      'machine_name' => 'PeopleOrganizations',
      'source_vocabulary' => '4',  // D6 Vocabulary ID
      'destination_vocabulary' => 'non_journalists',
    ),
  );

  $common_vocabulary_arguments = $common_arguments + array(
    'soft_dependencies' => array('NBUser'),
  );
  foreach ($vocabulary_arguments as $arguments) {
    $arguments += $common_vocabulary_arguments;
    Migration::registerMigration($arguments['class_name'], $arguments['machine_name'],
      $arguments);
  }
}

class NBPeopleOrganizationsTermMigration extends DrupalTerm6Migration {
  public function __construct(array $arguments) {
    parent::__construct($arguments);
  }

  /**
   * Implementation of Migration::prepareRow().
   *
   * Need to redirect terms into either Journalists or Non-Journalists vocabularies
   * based on whether term is listed in nb_migrate_journalists table.
   *
   * @param $row
   */
  public function prepareRow($row) {
    if (parent::prepareRow($row) === FALSE) {
      return FALSE;
    }
    static $journalists = array();
    if (!$journalists) {
      $sql = "SELECT * FROM nb_migrate_journalists";
      $result = db_query($sql);
      foreach ($result as $record) {
        $journalists[] = $record->tid;
      }
    }

    // Check to see if term is in $journalists.  If not, leave it as
    // non_journalists (as defined in migration registration), otherwise
    // change it to journalists.
    if (in_array($row->tid, $journalists)) {
      $row->destination_vocabulary = 'journalists';
    }
  }
} 

When I try to run this particular migration (via drush migrate-import), I get the following error:

Migration failed with source plugin exception: SQLSTATE[23000]: Integrity constraint violation: 1052 Column 'tid' in where clause is ambiguous

which doesn't make sense to me, since I haven't modified the query to select the terms from D6 in the first place. If I use the default DrupalTerm6Migration class, it works fine. In fact, the error hits before prepareRow() is called (and I've verified that my class is registered by looking in the migrate_status table). I've tried to track in down in my debugger, but haven't had any luck yet.

Am I doing something wrong that's causing this error?

Thanks.