fluent-plugin-kafka kafka_group error="key not found"

[ShellongC]

Recently, I use fluentd to consume kafka message and then write it to hdfs.Here is the configuration below:

@type kafka_group brokers _borker:port_ consumer_group _group_name_ topics _topic_name_ format json start_from_beginning false @type webhdfs namenode _namenodeaddress_ standby_namenode _standby_namenode_ path _hdfs_path_ username name @type json

while I start it.there occurred some error messages:
unexpected error during consuming events from kafka. Re-fresh events. error="key not found :"topic-name" Did you mean? "anthoer-topic-name"
and some stack info:
offset_manager.rb:199:infetch
offset_manager.rb:199:infetch_resolved_offsets
offset_manager.rb:194:inresolve_offset
So what does this mean? Each record should have a key?

[ShellongC]

version info:
fluent-plugin-kafka : 0.15.1
fluentd : 1.11.2
ruby: 2.6