GRN without material code

Hi,

Our client has created a PO without material code.

In account assignment category,K is given and corresponding GL account and cost center given.

When we try to make GR,system accepts and posted the value to that partcular account.

(As far i know,if no material code is there ,we have to give D in item category for services, so that service entry sheet can be made)

Please advice.