Issue with code

[rickprince]

I'm new to craft, so apologies if this is super simple. I'm having trouble getting this code working:

{% block content %}
    <ul>
    {% if block.images | length %}
        {% set thumb = {
          mode: 'fit',
          width: 550,
          height: 390,
          quality: 80,
        } %}
        {% for asset in block.images %}
            <li>
                <img src="{{ craft.retinafy.image(asset, 'thumb') }}" />
            </li>
        {% endfor %}
    {% endif %}
    </ul>
{% endblock %}

Error:

Argument 2 passed to Craft\Retinafy_RetinafyService::create2xImage() must be an instance of Craft\AssetTransformModel, null given, called in /my_system_path_here/craft/plugins/retinafy/services/Retinafy_RetinafyService.php on line 65 and defined

Thank you!