What did you do?
When the sns configuration has attributes referencing templates that does not exist Notify will fail but the error says that its because the topic_arn is blank.

Error message:

Invalid parameter: TopicArn Reason: An ARN must have at least 6 elements, not 1

What did you expect to see?
When the sns configuration has attributes referencing templates that does not exist the error should tell me that Notify failed because of this missing template, not something about the topic_arn.

• Alertmanager configuration file:

    - name: 'test'
      sns_configs:
        - topic_arn: "arn:aws:sns:us-west-2:123456:TestTopic"
          send_resolved: false
          sigv4:
            region: "us-west-2"
          attributes:
            tags: '{{ template "__unknown_template" .Alerts.Firing }}'