Base document exhcange rate cannot be use

Dear all,

I have a problem when I want to use base document exchange rate for creating AP Invoice.

Example :

Exchange rate as per 17-06-2009 : 9,000

Exchange rate as per 18-06-2009 : 10,000

I create Good Receipt PO posting date 17-06-2009.

A/P Invoice posting date 18-06-2009

For A/P Invoice I want to use the exchange rate base on (same with) GRPO's exchange rate.

I choose "Use Exchange Rate from Base Document" from document wizard.

But why this message :

"Base document exchange rate cannot be used

The row will be update using the target document exchange rate".

Anybody know why this happen ?

Best regards,

Surya