external operation storage location

Hi,
   I am generating a purchase requisiton for an external operation from a production order. This purchase req is a std purchase req and account assignment is F (production order). The line item does not have a storage location due to which the ME59N fails. Does anyone know how I can get the strorage location for this external operation purchase req to populate and how I can get the ME59N to work for purchase reqs of these kinds ?

Thanks