Invoice for Framework PO after validity end

Hi,

I have a framework PO created wrt a contract.

Validity end date of the contract as well as of the PO is 31.01.2013.

But system is allowing me to post invoice for the PO on 10.04.2013 i.e after validity end date , without throwing any kind of warning or error for the same.

Is there any customising so that system will give me either an error or a warning while posting the invoice.

Thanks

Soumya