PO price determination not update when delivery date changed

hi experts,

i have a problem when create PO.

I have a info record for a material, and the price is valid to 2015.12.31 and the price contol indicator is 2(delivery date)

when i create PO and put a delivery date 2015.8.20, then the system give the right price according to the price valid date.

But when i change the PO delivery date to 2016.8.20, the system will not get any message and save successfully.

for a correct result, cos in the delivery date 2016.8.20 there is no valid price, the system should give a erro message and can't save the PO

So, is there anybody know how to deal with this issue.

Thank you for all your helps.