Filter ruby exception occurred with no message

I encountered same issue with Ruby exception with no message.

following is logstash log info.

following is configuration info.

When I get output, the "index_postfix" can not be parsed as a variable.
following is output event es_index field:

This problem is happened after I restart logstash, but sometimes it works, sometimes it returns ruby exception.

logstash version is 6.2.3.
Anyone can help ? Many thanks!

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