Supplier Site not available while making Pay in Full

Hi Experts,

                 For making payment to a Standard Invoice, when we click on Actions Button Pay in Full, then in Payment work bench Supplier Name & number is defaulting from the Invoice work bench but the Supplier  Site it is not available . I.e Lov for Supplier Site is not available in payment work bench while making pay in full for a standard Invoice.

Note : Supplier type =Trade Creditors Ex ternal

Please advise