shopping cart error in process: Faulty account assignment

Dear Experts,

We have a issue with Catalogue shopping carts which results in Faulty account assignment error. While checking we found that the field unloading point is getting null and when we are manually entering the field unloading point then error is not there.

Please suggest why shopping cart is giving error even if unloading point is being cleared or what needs to be done so that this filed is not null.

Thanks.

Regards,

Abhi