Buyer Work Center Functionality not usable because of Category Error

We upgraded to R12 but are unable to use the Buyer Work Center functionality because of a category error when creating a purchase order from a requisition.  If any change, price, quantity, etc is made to the requisition after creating a purchase order in the buyer work center, the category, which is not locked as it is in forms, throws up an "invalid category" error.  This same requisiton can be auto created into a purchase order in forms without any errors and allows changes to be made.  No errors are seen when creating the requisition in iprocurement.  Requisitions created using