E1 9.1 Entering a 3 way Inventory PO match and recieving error that Blanks are invalid in G/L Accoun

Hi --

We are have set up inventory and are trying to voucher our test PO.  It has been received and there is a record in the F43121.  When we try to do a 3 way match in A/P, we are receiving the error that Blanks are invalid and the field highlighted is the Expense Account field.  The Line Type is S and is set up with and inventory code of Y... so why would 3-way match be requiring the expense account to be filled in.  We have verified that the DMAAs have been set up.  Is there a processing option in