lot sizing procedure

hi gurus ,

when creating material master for material type "fert"

I am getting the following error pl resolve urgently

*Enter a valid lot size (materials planning)*

*Message no. M3497*

*Diagnosis*

*The lot size specified in materials planning does not make sense here since quantity updating is not defined for the current material type in the plant specified.*

*Procedure*

*Choose a lot size for which the lot-sizing procedure "lot-for-lot order quantity" is defined.*

but I have maintained mrp type as PD and lot size as TB,the above error is received for this if I maintain EX system is accepting , but the earlier combination already exists for diff plants which are on live , pl come back at the earliest

thanks Regards

ramakrishna