Cannot re-order membership types using arros

Clicking on an arrow to change the order of Membership Types in the list at http://drupal6.demo.civicrm.org/civicrm/admin/member/membershipType?reset=1 causes this error:

Sorry. A non-recoverable error has occurred.
Request signature is invalid

Backtrace:
/var/www/drupal7.civicrm.org/svn/civicrm_stable/CRM/Core/Error.php, backtrace, 288
/var/www/drupal7.civicrm.org/svn/civicrm_stable/CRM/Utils/Weight.php, fatal, 420
, fixOrder,
/var/www/drupal7.civicrm.org/svn/civicrm_stable/CRM/Core/Invoke.php, call_user_func, 203
/var/www/drupal.civicrm.org/public/sites/drupal6.demo.civicrm.org/modules/civicrm/drupal/civicrm.module, invoke, 352
, civicrm_invoke,
/var/www/drupal.civicrm.org/public/includes/menu.inc, call_user_func_array, 349
/var/www/drupal.civicrm.org/public/index.php, menu_execute_active_handler, 17

Clicking Edit & using the Order field to rearrange membership types works.