If I don't post PO receipt for prepay, then I can't invoice the prepay. Why?

Hi All,

I would think that after I confirmed a PO with prepay, I can go straight to invoice the prepay.  I don't have to post a Product receipt; because prepay is not the product to receive; but when I try to post the invoice for prepay, the line is empty.  Am I missing a set up?

The screenshot below show the invoice line empty. 

eImage may be NSFW.
Clik here to view.