Help with GR for Inbound Delivery with MIGO

Hi,

I'm trying to post goods receipt in R/3 4.0 for an inbound delivery that exists in the system with transaction MIGO. After inputing the inbound delivery number, I get an error message that tells me the PO number doesn't exist. It is reading the sales document number as if it were the PO number.

The Sales order that the delivery was created from contains the correct PO number that's referenced to. The inbound delivery IDoc also contains the correct PO number in the item level. Does anyone know why in MIGO it's reading the sales order number as the PO number? Thanks.

Please help.