Grafana Alert, Certificate error

Getting below error on Grafana logs for alert while using Prometheus as data source for email alerting.

logger=alerting.evalContext ruleId=3 name=“Some Alert” error=“tsdb.HandleRequest() error Post “https://prometheus.{}/query_range”: x509: certificate signed by unknown authority” changing state to=alerting

Promethus and Grafana (7.3.3) are installed on same server box and test alert is working fine from Grafana UI.