Checkboxes show empty in Netlify forms

A form which submits all its items properly to the Netlify forms shows the checkboxes values as empty, no matter if the checkbox has been checked or not.

HTML/Nunjucks partial:

    {# checkboxes #}
    <fieldset class="check-container col-12 px-md-0">
      {# subscribe to newsletter #}
      <div class="form-check">
        <input
          type="checkbox"
          id="subscribeCheck"
          name="subscribeCheck"
          value="newsletterSubscription"
          class="form-check-input"
        />
        <label class="form-check-label" for="subscribeCheck">
          
        </label>
      </div>
      {# subscribe to newsletter end #}

      {# 18+ #}
      <div class="form-check">
        <input
          type="checkbox"
          id="rulesCheck"
          name="rulesCheck"
          value="rulesCheck"
          class="form-check-input"
          required
        />
        <label class="form-check-label" for="rulesCheck">
          
        </label>
      </div>
      {# 18+ end #}
    </fieldset>
    {# checkboxes end #}

Is there a way to force a true/false value for the checkbox data?