Auto-increment Ref# in Journal Voucher

Hello, Accounting -->Journal Entries-->Journal Vouchers-->Create How Can I set my REF# field Auto-incremented ,in Pg admin admin table Name="Account_voucher" Column="Reference" I want this Reference to be auto-incremented I went to it's properties and tried to set it's data type "Bgiserial "autoincrementing eight-byte integer but since it's an existing column it's very restricted an bigserial can't be selected. what should I do now should I create a new column with bigserial data type I tried it too but it's crashing the Journal Voucher Form.Kindly helo me in it. Regards