I added field note to invoice report inside tax table. i want tax table to show up only if field note filled up. 

getting this error.  but field does exist and works wine if i don't use last xpath with position=replace.

QWebException: "'NoneType' object has no attribute 'note'" while evaluating

'not t.note'

<openerp>
    <data>

        <template id="report_invoice_document" inherit_id="account.report_invoice_document">


            <xpath expr="//span[@t-field='t.amount']/../../../../thead/tr" position="inside">
                <th class="text-left">Note</th>

            </xpath>

            <xpath expr="//span[@t-field='t.amount']" position="after">
                    <span t-field="t.note"/>
            </xpath>
          <xpath expr="//span[@t-field='t.amount']/../../../.." position="replace">
                <th t-if="not t.note"/>
            </xpath>


        </template>
    </data>
</openerp>