Uninstall "drop tall tables" fails when uninstalling

I set the value: $uninstall = true; in the file uninstall.civicrm.php. Then I tried to uninstall CiviCRM beta 2 under Joomla. I received the following error page:

Sorry. A non-recoverable error has occurred.
DB Error: constraint violation

Database Error Code: Cannot delete or update a parent row: a foreign key constraint fails, 1217

Return to home page.

Error Details:

Array
(
[callback] => Array
(
[0] => CRM_Core_Error
[1] => handle
)

[code] => -3
[message] => DB Error: constraint violation
[mode] => 16
[debug_info] => DROP TABLE IF EXISTS civicrm_contribution [nativecode=1217 ** Cannot delete or update a parent row: a foreign key constraint fails]
[type] => DB_Error
[user_info] => DROP TABLE IF EXISTS civicrm_contribution [nativecode=1217 ** Cannot delete or update a parent row: a foreign key constraint fails]
[to_string] => [db_error: message="DB Error: constraint violation" code=-3 mode=callback callback=CRM_Core_Error::handle prefix="" info="DROP TABLE IF EXISTS civicrm_contribution [nativecode=1217 ** Cannot delete or update a parent row: a foreign key constraint fails]"]
)