Get the weight of a shipment inside email : issue with placeholder

Commerce
#1

Hi,

I want to print the shipment total weight on the order-to-merchant.twig template but the {{ shipment.total_weight }} placeholder stay empty during parsing.
I insert it like this :

{% for shipment in shipments %}
    <tr>
        <td valign="top" style="padding: 0;" class="mobile-wrapper">
            <!-- LEFT COLUMN -->
            <table cellpadding="0" cellspacing="0" border="0" width="47%" style="width: 47%;" align="left">
                <tr>
                    <td style="padding: 0 0 10px 0;">
                        <table cellpadding="0" cellspacing="0" border="0" width="100%">
                            <tr>
                                <td align="left" style="font-family: Arial, sans-serif; color: #333333; font-size: 16px;">
                                    {% if shipment.method.id %}{{ shipment.method.name }}{% else %}{{ lex('commerce.shipping') }}{% endif %} - {{ shipment.total_weight }}
                                </td>
                            </tr>
                        </table>
                    </td>
                </tr>
            </table>
            <!-- RIGHT COLUMN -->
            <table cellpadding="0" cellspacing="0" border="0" width="47%" style="width: 47%;" align="right">
                <tr>
                    <td style="padding: 0 0 10px 0;">
                        <table cellpadding="0" cellspacing="0" border="0" width="100%">
                            <tr>
                                <td align="right" style="font-family: Arial, sans-serif; color: #333333; font-size: 16px;">{{ shipment.fee_formatted }} TTC</td>
                            </tr>
                        </table>
                    </td>
                </tr>
            </table>
        </td>
    </tr>
{% endfor %}

What did I miss ?