${tag_suffix} config is not available

[Gourds]

Now this ${tag_suffix[4]} config is not available。What is wrong with my configuration

<source>
  @type kafka
  brokers 10.17.0.112:9092,10.17.0.226:9092,10.17.0.211:9092
  topics prod_test
  #offset_zookeeper 10.17.0.112:2181,10.17.0.226:2181,10.17.0.211:2181
  #offset_zk_root_node  /zookeeper/data
</source>

<match debug.**>
  type stdout
</match>

<source>
  type debug_agent
  bind 127.0.0.1
  port 24230
</source>

##################################################################################
<match **>
      type elasticsearch
      #type_name logics.shard
      host data1.elasticsearch.taiqa.net
      port 9200
      logstash_format true
      logstash_prefix bilogs
#      logstash_dateformat logics-${tag_parts[-1]}.%Y.%m.%d
      logstash_dateformat logics-${tag_suffix[4]}.%Y.%m.%d
      time_key time
      flush_interval 10s
      request_timeout 15s
      reload_on_failure true
      reconnect_on_error true
</match>