Error with customer purchase order number and print format


I was using custom print format, which worked with no issuess. Sometime after version 10.1.12 it started to act weird.
I noticed that in the table when there is no purchase order number value, system leaves empty field instead “null” value. Which is apparently also making problem with my custom format.

Is there anything I can do to fix this or is it a bug?

Piece of my custom print format that currently does not work as it should.

 {%if doc.po_no is not none-%}
            <div class="row" data-fieldname="po_no" data-fieldtype="Data" id="narociloKupca">
                <div class="col-xs-12 text-left">št. naročila kupca: <b>{{doc.po_no}}</b> </div>

running latest on master branch.

Thank you

Try using {{ cstr(doc.po_no) }}