Freemarker sample to iterate the portletPreferences variable on a Liferay Application Display Template

sample.ftl

<#--
portletPreferences: Represents the preferences of the application that are used to display content 
through this template. Use the map API of the selected template language to get the values of the 
preferences.
-->

<#if portletPreferences?has_content>
    <h1>Portlet Preferences</h1>

    <ul>
        <#list portletPreferences?keys as key>
            <#assign values = portletPreferences[key] />
            
            <#if values?has_content>
                <li>${key}:
                    <ul>
                        <#list values as value>
                            <li>${value}</li>
                        </#list>   
                    </ul>
                </li>
            </#if>
        </#list>
    </ul>
</#if>

This doesn't work due to freemarker.template.TemplateModelException: Getting the number of items or enumerating the items is not supported on this method+sequence (wrapper: f.e.b.SimpleMethodModel) value.

Edit
Ye, portletPreferences?keys returns a map of class members...