INVALID: Attempting to add a domain to Zerigo fails

Adding the Zerigo addon with a domain that already exists on Zerigo gives the following error, but the client only sees "Internal Server Error" so they don't have enough information to know what's failing.

2009-12-28T12:26:21-08:00 core.12940 core[8492]: RuntimeError processing http://heroku.com/apps/jeffrey-wilcke/domains/
        RuntimeError: Unknown Exception: Domain is already attached to another account
        /mnt/home/core/lib/zerigo_api.rb:341:in `api_error_to_exception'
        /mnt/home/core/lib/zerigo_api.rb:63:in `add_domain'
        /mnt/home/core/app/models/domain_name.rb:293:in `zerigo_dns_update'
        /mnt/home/core/app/models/domain_name.rb:32:in `validate'
        /home/core/vendor/bundler_gems/gems/activerecord-2.3.4/lib/active_record/validations.rb:1099:in `valid_without_callbacks?'
        /home/core/vendor/bundler_gems/gems/activerecord-2.3.4/lib/active_record/callbacks.rb:315:in `valid?'
        /home/core/vendor/bundler_gems/gems/activerecord-2.3.4/lib/active_record/validations.rb:1077:in `save_without_dirty'
        /home/core/vendor/bundler_gems/gems/activerecord-2.3.4/lib/active_record/dirty.rb:79:in `save_without_transactions'
        /home/core/vendor/bundler_gems/gems/activerecord-2.3.4/lib/active_record/transactions.rb:229:in `send'
        /home/core/vendor/bundler_gems/gems/activerecord-2.3.4/lib/active_record/transactions.rb:229:in `with_transaction_returning_status'
        /home/core/vendor/bundler_gems/gems/activerecord-2.3.4/lib/active_record/connection_adapters/abstract/database_statements.rb:136:in `transaction'
        /home/core/vendor/bundler_gems/gems/activerecord-2.3.4/lib/active_record/transactions.rb:182:in `transaction'
        /home/core/vendor/bundler_gems/gems/activerecord-2.3.4/lib/active_record/transactions.rb:228:in `with_transaction_returning_status'
        /home/core/vendor/bundler_gems/gems/activerecord-2.3.4/lib/active_record/transactions.rb:196:in `save'
        /home/core/vendor/bundler_gems/gems/activerecord-2.3.4/lib/active_record/transactions.rb:208:in `rollback_active_record_state!'
        /home/core/vendor/bundler_gems/gems/activerecord-2.3.4/lib/active_record/transactions.rb:196:in `save'
        /mnt/home/core/app/controllers/domain_names_controller.rb:14:in `add'
        /home/core/vendor/bundler_gems/gems/actionpack-2.3.4/lib/action_controller/mime_responds.rb:106:in `call'
        /home/