prometheus config alertmanager error #758

When starting up the containers, prometheus exits with the error:

Error loading config (--config.file=/etc/prometheus/prometheus.yml)" file=/etc/prometheus/prometheus.yml err="parsing YAML file /etc/prometheus/prometheus.yml: expected Alertmanager api version to be one of [v2] but got v1

I edited the prometheus.yml and changed api_version from v1 to v2 and that fixed it.