Unplanned Delivery account requires a CO object -why it is giving the error

HI,

When trying to post a MIRO invoice for PO 45784 we are getting the attached error message ..

Account 580400 requires an assignment of CO object

580400 is unplanned delivery cost account.I gave the solution to the above issue.But client is asking why this error we didnot get for before postings,why we are getting now only.what could be the reason.why it did not give any error before.why we got the error for this transaction.How can i answer for this question.

Kindly advice me how to answer for this.

Thanks

Sunitha