Error when converting 3rd Party PReq to Schd. Agrmt. Schedule Line

I created a 3rd party purchase requisition from a 'TAS' sales order item. My purchasing item catagory is 'S' and the account assignment is 'X'. I also have created a schedule agreement with the same item catagory and account assignment settings. The schedule agreement and fixed vendor are referenced in the purchase requisition, however when i go to ME38 and try to create a delivery schedule with reference to the purchase req, i get error message 06251 "Requisition ########, item #####, assigned to other order account"

Does anyone know how to fix this problem? I'm not sure what the other order account could be.