Validation not allowing to book the cost for particular wbs element at the time of MIGO.

Hi Gurus,

I have wrote validation in Cost accounting for line item   1. Prerequisite is   PRPS-PRART = 'IP'

                                                                                  2.Check is COBL-SAKNR IN SET_IP OR COBL-HKONT IN SET_IP...

                                                                                  3.Message is Assign Proper WBS element.

Then i have maintained Stock GL's and Inventory GL's in GS01.

Then i have activated validation in OKC7.

Based on the above conditions i have created PO it's allowing to book the cost.

Then i want to create MIGO it's not allowing to book the cost, it's throwing error message assign proper wbs element.

  This requirement is for control wrong postings...

please suggest.....