Can't delete contribution records in CiviCRM

[Upperholme]

Recently got this installed and running on a second site, and have spotted a couple of issues:

I put through a few dummy transactions in Civi to make sure everything was doing what it was supposed to. When I try and delete any of these it doesn't work. Nothing on-screen, but the log reports a foreign key constraint:

[user_info] => DELETE FROM civicrm_contribution WHERE ( civicrm_contribution.id = 2 ) [nativecode=1451 ** Cannot delete or update a parent row: a foreign key constraint fails (vcare_2.civicrm_account_invoice,
CONSTRAINT FK_civicrm_account_invoice_contribution_id FOREIGN KEY (contribution_id) REFERENCES civicrm_contribution (id))]

Have I set CiviXero up incorrectly?