Simple alert for watching multiple hosts that could be down

Grafana newcomer here, using 9.2.0-beta1.

I have several hosts that I monitor (which are called clients here), and they produce data at least once per hour. I want to get an alert if any of those clients has not produced data within that one-hour period.

This sounds quite simple but I’m not getting there. I have an Elasticsearch query that gives me a list of unique counts of client labels over a longer period of time (e.g., 3 hours), grouped by 1 hour buckets. These buckets become 0 if there suddenly is no instance of that label anymore:

All I need now is an alert if any of those series’ last value (or min value) becomes 0.

So I try math:

But when I preview the alerts, I get this error:

invalid format of evaluation results for the alert definition : frame cannot uniquely be identified by its labels: has duplicate results with labels {}

What does this mean?

The result of my expression looks meaningful to me: