Collapsable card - title card not working

Hi,
I want to use a collapsable card with a title_card. However it doesnt dispplay the card it only displays “Toggle”. What is wrong?

type: custom:collapsable-cards
title_card:
  - type: custom:mushroom-template-card
    primary: McFit
    secondary: '{{ states(''sensor.mcfit'') }} %'
    icon: mdi:dumbbell
    icon_color: yellow
cards:
  - type: custom:stack-in-card
    cards:
      - type: custom:mini-graph-card
        entities:
          - entity: sensor.mcfit
            name: McFit
            color: yellow
            unit: '%'
        smoothing: true
        hours_to_show: 12
        line_width: 3
        font_size: 50
        animate: false
        lower_bound: 0
        upper_bound: 100
        points_per_hour: 2
        show:
          name: true
          icon: false
          state: true
          legend: false
          fill: fade
buttonStyle: 'background-color: var(--card-background-color);'