Modular page content custom order

I am trying to get modular page content in a custom order, but so far, no luck. Content is displayed, but in a wrong order.

Here is a code of modular.html.twig:

{% for module in collection %}
  <div class="row clearfix">
    <h2>{{ module.title }}</h2>
    {{ module.content }}
  </div>
{% endfor %}

Here is a modular page content:

content:
    items: '@self.modular'
    order:
        custom:
            - _contentA
            - _contentB
            - _contentC

For example, my content wil be displayed in that order contentB, contentC, contentA, but not in which I have specifically configured.

What may be the problem?

The custom order is based on the 'slug' of the modular pages.. that is usually based on the folder names of those modular pages. So do those match the ones you've specified in your custom order?

Will try it with the slug defined.

Thank you, rhukster! Works!