Alert list panel: show external Alertmanager alerts

I am trying to use an “Alert list” panel to display pending and firing alerts of an external Alertmanager. It is working perfectly with the included Alertmanager and displays Grafana alerts but not with the external one.

I have the external Alertmanager in my data Sources, but in the Alert list panel I can only choose “Prometheus” as alternative data source and selecting that leads to no alerts being displayed.

Could anybody point me in the right direction?