Consistent error message on all updates after upgrading from 6.x-1.8 to 7.x-1.0-rc1

After an otherwise successful site upgrade from Drupal 6.26 to Drupal 7.12, I am getting this error message consistently whenever I run update.php:

"Notice: Undefined property: stdClass::$admin_description in page_manager_page_build_subtask() (line 204 of [...] sites\all\modules\ctools\page_manager\plugins\tasks\page.inc)."

The error is occurring here:

$subtask = array(
'name' => $page->name,
'admin title' => check_plain($page->admin_title),
==> 'admin description' => filter_xss_admin($page->admin_description),
'admin summary' => 'page_manager_page_admin_summary',
'admin path' => $page->path,
'admin type' => t('Custom'),
'subtask' => $page,
'operations' => $operations,
'operations include' => array(
'file' => 'page.admin.inc',
'path' => drupal_get_path('module', 'page_manager') . '/plugins/tasks',
),
'single task' => empty($page->multiple),
'row class' => empty($page->disabled) ? 'page-manager-enabled' : 'page-manager-disabled',
'storage' => $page->type == t('Default') ? t('In code') : $page->type,
'disabled' => !empty($page->disabled),
// This works for both enable AND disable
'enable callback' => 'page_manager_page_enable',
);

Any idea why the admin_description property is missing from the page?

-- Kari