Error while nesting mini-graph-card under horizontal-stack in 0.81

Configuration
1

This works fine:

- id: weather-view
  cards:
  - id: temperature
    type: custom:mini-graph-card
    entity: sensor.rfw_outside

This on the other hand generates an error Cannot read property ‘states’ of undefined.

- id: weather-view
  cards:
    - id: stack
      type: horizontal-stack
      cards:
        - id: temperature
          type: custom:mini-graph-card
          entity: sensor.rfw_outside

Any ideas why? I’m using HA v0.81.