Failed to query data EOF when using alerting for my datasource with backend

I meet some problems when using alerting for my datasource with backend.
When clicking “preview” button in “create rule”, I will reveive the following error:

Failed to evaluate queries and expressions: [plugin.downstreamError] failed to query data: Failed to query data: rpc error: code = Unavailable desc = error reading from server: EOF

I have enabled “alerting” in “plugin.json”, and followed the instruction of alerting in Grafana doc.
This dataource can be normally used in Grafana Panel. Only exception in Grafana Alerting.