Unapplied Receipt account (GL) shows Debit balance for a customer

Following accounting takes place on entering receipt:

Cash                          Dr

Unapplied Receipt A/c        Cr

Receipt applied to invoice: Unapplied Receipt A/c      Dr

Receipt is fully applied: Unapplied Receipt A/c balance becomes  NIL

There doesnt seem to be a way the unapplied receipt A/c balance can be net Debit.

However one of our customers have a net Dr balance in Unapplied Receipt A/c.