Ruby exception occurred:Missing Valuefier handling

ruby {
		  code => "event.set('timestamp', event.get('@timestamp').time.localtime + 7*60*60)"
}	

[2019-11-28T18:23:24,424][ERROR][logstash.filters.ruby ] Ruby exception occurred: Missing Valuefier handling for full class name=org.jruby.RubyEnumerator, simple name=RubyEnumerator

I use Ruby filter, no matter whether there is any content in the code, it will report an error