To make the “Destination subinventory” field on PO form Distribution block a required field

To make the “Destination subinventory” field on PO form Distribution block a required field

I want to make the “Destination subinventory” field (PO_DISTRIBUTIONS.DESTINATION_SUBINVENTORY) mandatory on PO form Distribution block (PO_DISTRIBUTIONS).
I tried on the following things.
1. Property : Item-->PO_DISTRIBUTIONS.DESTINATION_SUBINVENTORY-->REQUIRED
2. Property : Item-->PO_DISTRIBUTIONS.DESTINATION_SUBINVENTORY-->REQUIRED (APPLICATIONS COVER)

The fields seems to be made mandatory(colour turned YELLOW). But, system allow me to save and Approve the PO without providing a value to this field. If I click on the “Destination subinventory” field at PO form Distribution and then try to save the PO without providing a value, system is displaying a message that the field is mandatory. Could any body please help me to make me fix this so that the system will not allow the user to save and Approve the PO without providing a value to this field?