Elapsed filter - keeps crashing

[Tommie]

Can anyone please help me with the following:
This logstash config is working fine (only with the elapsed part commented):

filter {

    csv { 
        separator => "|"
        columns => [ "datum","level","uniek","message","teller" ]
    }
date {
        match => [ "datum", "yyyy-MM-dd HH:mm:ss.SSSS" ]
	}

	if "Proces is gestart" in [message] { 
		mutate { 
			add_tag => [ "taskStarted"] 
		}
	}
	if "Proces is geeindigd" in [message] {
		mutate { 
			add_tag => [ "taskTerminated"] 
		}
	}
#	elapsed {
#		unique_id_field => "uniek"
#		start_tag => "taskStarted"
#		end_tag => "taskTerminated"
#		timeout => 10
#	}
}

When I uncomment the elapsed part (the most important part!) it crashes.

Error message is below

An unexpected error occurred! {:error=>#<LogStash::Error: Don't know how to handle Java::JavaLang::IllegalStateException for PipelineAction::Create<main>>, :backtrace=>["org/logstash/execution/ConvergeResultExt.java:129:in create'", "org/logstash/execution/ConvergeResultExt.java:57:in add'", "C:/_ELK/logstash/logstash-core/lib/logstash/agent.rb:355:in `block in converge_state'"]}