Fatal error when exporting from CiviMember

Hi,
In the sandbox: testing the export functionality in CiviMember, I did the following:

1. Find members
2. Click on Export members
3. Click on Export primary Fields

When I click on "continue", I receive the following error:
Fatal error: Class 'CRM_Contribute_BAO_Contribution' not found in /var/www/drupal-6.civicrm.org/public/sites/sandbox.civicrm.org/modules/civicrm/CRM/Export/BAO/Export.php on line 237

When I do the same thing in our internal sandbox site, which is running 2.1 alpha 3, I receive the following errors:
backTrace

/var/www/afscsandbox/sites/afscsandbox.starcafecrm.org/modules/civicrm/CRM/Core/Error.php, backtrace, 138
, handle,
/var/www/afscsandbox/sites/sitename/modules/civicrm/packages/PEAR.php, call_user_func, 912
/var/www/afscsandbox/sites/sitename/modules/civicrm/packages/DB.php, PEAR_Error, 966
/var/www/afscsandbox/sites/sitename/modules/civicrm/packages/PEAR.php, DB_Error, 574
/var/www/afscsandbox/sites/sitename/modules/civicrm/packages/DB/common.php, raiseError, 1903
/var/www/afscsandbox/sites/sitename/modules/civicrm/packages/DB/mysql.php, raiseError, 898
/var/www/afscsandbox/sites/sitename/modules/civicrm/packages/DB/mysql.php, mysqlRaiseError, 327
/var/www/afscsandbox/sites/sitename/modules/civicrm/packages/DB/common.php, simpleQuery, 1216
/var/www/afscsandbox/sites/sitename/modules/civicrm/packages/DB/DataObject.php, query, 2390
/var/www/afscsandbox/sites/sitename/modules/civicrm/packages/DB/DataObject.php, _query, 1583
/var/www/afscsandbox/sites/sitename/modules/civicrm/CRM/Core/DAO.php, query, 144
/var/www/afscsandbox/sites/sitename/modules/civicrm/CRM/Core/DAO.php, query, 852
/var/www/afscsandbox/sites/sitename/modules/civicrm/CRM/Export/BAO/Export.php, executeQuery, 212
/var/www/afscsandbox/sites/sitename/modules/civicrm/CRM/Export/Form/Select.php, exportComponents, 211
/var/www/afscsandbox/sites/sitename/modules/civicrm/CRM/Core/StateMachine.php, postProcess, 166
/var/www/afscsandbox/sites/sitename/modules/civicrm/CRM/Core/QuickForm/Action/Next.php, perform, 63
/var/www/afscsandbox/sites/sitename/modules/civicrm/packages/HTML/QuickForm/Controller.php, perform, 203
/var/www/afscsandbox/sites/sitename/modules/civicrm/packages/HTML/QuickForm/Page.php, handle, 103
/var/www/afscsandbox/sites/sitename/modules/civicrm/CRM/Core/Controller.php, handle, 232
/var/www/afscsandbox/sites/sitename/modules/civicrm/CRM/Core/Invoke.php, run, 197
/var/www/afscsandbox/sites/sitename/modules/civicrm/drupal/civicrm.module, invoke, 314
, civicrm_invoke,
/var/www/afscsandbox/includes/menu.inc, call_user_func_array, 346
/var/www/afscsandbox/index.php, menu_execute_active_handler, 18

Can you help?

Thanks,
Cynthia