urlencode does not work

[pycada]

I would like to encode the value of t in the url with urlencode. It doen't work. Do you have an idea why ?
thank you.

{% for t in liste_theme_key %}
                <div class="theme">
                  {% with 'accueil/img/'|add:t|add:'.png' as png_url %}
                        <a href="/theme/{{t|urlencode}}"><img class="image_theme" src="{% static png_url  %}" style="width:270px; height: 170px; margin-top:17x;"/></a>
                  {% endwith %}
                </div>
{% endfor %}