toString() fails on foreach ({ternary} as $foo)

  foreach (isset($_SESSION['user_overview_filter']) ? $_SESSION['user_overview_filter'] : array() as $filter) {

is line 3014 of Drupal HEAD's modules/user/user.module at the moment. API module tries to call ->toString() on it

30.1026 99177056 18. api_documentation_loop() /home/drumm/drupal/6/sites/drumm-x200.api-6/modules/api/parser.inc:102
30.9871 99649860 19. PGPClass->toString() /home/drumm/drupal/6/sites/drumm-x200.api-6/modules/api/parser.inc:160
30.9873 99651264 20. PGPBody->toString() /home/drumm/drupal/6/sites/drumm-x200.api-6/modules/grammar_parser/engine/grammar_parser.object.inc:662
30.9880 99652696 21. PGPForeach->toString() /home/drumm/drupal/6/sites/drumm-x200.api-6/modules/grammar_parser/engine/grammar_parser.list.inc:1667

In the end, it seems $element is an empty string when this is tried

        case 'value':
          $string .= $space . $element->toString();