Alfanumeric voucher number not allowed in the same journal

I am trying to enter GL journal or AR payments, if the voucher nuber contains alfanumeric characters when you try to add the second voucher with alfa numeric voucher number in the same journal, you get an erro massage “Warning: Invalid specification of Voucher” and the voucher number changes to the first entered voucher in the journal. This also happens if the voucher number starts with zero eg. if you enter a voucher 0001 you cannot add 0002 in the ame journal advise please