Applying Prepayment - A currency to convert from is required to retrieve exchange rate information? (Accounts Payable)

Good Afternoon All,

I previously created a vendor prepayment invoice.  After the item shipped, i invoiced the PO and later tried to apply the prepayment to the PO invoice. 

However, i got this error message "A currency to convert from is required to retrieve exchange rate information"

Please what can i do to fix this issue so i can apply the prepayment?

Thanks.