How to create an alert in case errors are logged?

I want Grafana to notify us about any error which was logged. I am struggling with the query, however:

sum(count_over_time({app="my-app"} | json | Level="Error" or Level="Critical" or Level="Fatal"[1m])) > 0

When I try to preview alerts, I get this error:

invalid format of evaluation results for the alert definition Failures: looks like time series data, only reduced data can be alerted on.

What do I need to make data reduced?