I'm using an adapted version of Odoo 8 and trying to validate a Customer Invoice.

When I press the Validate button, I get the above error message.

I'm at a loss to see which company my Period belongs to. If I go to the Invoicing module and look at the Period, I see a Period Name, Code, Start and End of Period, and a check box for Opening/Closing Period but that's it.

Or am I missing something very obvious?