how can I send alert and show top 5 backend server name

[YTS85205107]

My current ELK is collecting haproxy log
I want to send an alert to slack when http status code 500~599 exceeds 10 transactions in one minute, and display the backend server name, but backend server name of alert show MISSING VALUE

rule yaml below:

name: "haproxy 5xx log"
type: frequency
index: ha-*
num_events: 10
timeframe:
  minutes: 1
realert:
  minutes: 0
#aggregation:
#  minutes: 1

query_timezone: "Asia/Taipei"
use_local_time: true
top_count_keys: ["backend_name.keyword"]
top_count_number: 5

timestamp_format: "%Y-%m-%d %H:%M:%S %Z%z"

filter:
  - range:
      http_status_code:
        from: 500
        to: 599

alert:
  - "slack"

slack:
slack_webhook_url: "https://hooks.slack.com/services/xxx/xxx/xxx"
slack_proxy: "http://10.10.10.10:12345"
slack_username_override: ""
slack_channel_override: "#haproxy"

alert_text_type: alert_text_only
alert_text_args: ["client_ip", "request_header_host", "request", "http_status_code", "timestamp", "@timestamp", "top_count_keys.backend_name.keyword", "top_count_keys", "top_backend_names.keyword"]
alert_text: |
  Site: kenkenken
  Backend Server: {6}
  Backend Server: {7}
  Backend Server: {8}
  Alert time: {4}
  Alert time: {5}

Thanks

[YTS85205107]

Sorry, this problem has been solved, please ignore it, thanks.