purchase invoice with back order from supplier

[HyunWoo Jo]

hi,
i am sorry if it's a duplicated question.
i have been searching, but i wasn't able to find answer.

i created a purchase order for 10 items from supplier A.
i received 6 items out of 10, and 4 items are on back order at supplier.
but it doesn't generate a Purchase Invoice automatically,
did i do something wrong?

i figured that if i receive entire items on the PO, then it generates  
Purchase Invoice automatically.

what should i do?

thank you in advance.

hyun