When I try to generate an invoice based on purchase order, it gives me this error:

Integrity Error

The operation cannot be completed, probably due to the following:
- deletion: you may be trying to delete a record while other records still reference it
- creation/update: a mandatory field is not correctly set

[object with reference: account_id - account.id]

I have set up accounts on the product category to which purchased products belong and supplier has his own account properties. I really dont know which account the system wants. The invoicing process of the purchase order is based on received shipments.