Error while sending QUERY packet. PID=881 when using cache key

Question

If I do:

{% set cacheKey = block.id %}
{% cache using key cacheKey if craft.app.config.general.cache %}
    {% for item in items %}

        <div class="swiper-slide ais-hits--item" data-id="{{ item.id }}">

            {% set object = (block.contentType == 'events') ? item.show.one : item %}

            {% include '_blocks/blockCard/blockCard' with {
                content: object,
                blockId: block.id
            } %}

            {% include '_blocks/blockCardPreview/blockCardPreview' with {
                content: object,
                blockId: block.id
            } %}

        </div>


    {% endfor %}
{% endcache %}

I get an error saying

PHP Warning – yii\base\ErrorException Error while sending QUERY packet. PID=881

If I remove the using key cacheKey part it works. But of course al the sliders have the same content.