Nextflow output value & Snakeyaml failure

.nextflow.log

Jun-19 16:03:13.939 [Actor Thread 4] WARN  nextflow.processor.TaskContext - Cannot serialize context map. Cause: java.lang.IllegalArgumentException: type cannot be null. -- Resume will not work on this process
Jun-19 16:03:13.942 [Actor Thread 4] DEBUG nextflow.processor.TaskContext - Failed to serialize delegate map items: [ 
  'x':[null] = [firstName:Jane, lastName:Doe]
  '$':[java.lang.Boolean] = true
  'task':[nextflow.processor.TaskConfig] = [process:foo, index:2, echo:false, validExitStatus:[0], maxRetries:1, maxErrors:3, shell:[/bin/bash, -ue], executor:local, name:foo (2), cacheable:true, errorStrategy:TERMINATE, workDir:/Volumes/LG_01_WD3TB/tmp/preqctest/work/85/8e1bdbe341d5644034fd3e24426884, hash:858e1bdbe341d5644034fd3e24426884]
  'paramsFilePath':[java.io.File] = /Volumes/LG_01_WD3TB/tmp/preqctest/work/85/8e1bdbe341d5644034fd3e24426884/params.yaml
  'yaml':[org.yaml.snakeyaml.Yaml] = Yaml:94182769
]
com.esotericsoftware.kryo.KryoException: java.lang.IllegalArgumentException: type cannot be null.
Serialization trace:
multiRepresenters (org.yaml.snakeyaml.representer.Representer)
representer (org.yaml.snakeyaml.Yaml)
        at com.esotericsoftware.kryo.serializers.ObjectField.write(ObjectField.java:82)
        at com.esotericsoftware.kryo.serializers.FieldSerializer.write(FieldSerializer.java:495)
        at com.esotericsoftware.kryo.Kryo.writeObject(Kryo.java:523)
        at com.esotericsoftware.kryo.serializers.ObjectField.write(ObjectField.java:61)
        at com.esotericsoftware.kryo.serializers.FieldSerializer.write(FieldSerializer.java:495)
        at com.esotericsoftware.kryo.Kryo.writeClassAndObject(Kryo.java:599)
        at com.esotericsoftware.kryo.serializers.MapSerializer.write(MapSerializer.java:95)
        at com.esotericsoftware.kryo.serializers.MapSerializer.write(MapSerializer.java:21)
        at com.esotericsoftware.kryo.Kryo.writeClassAndObject(Kryo.java:599)
        at com.esotericsoftware.kryo.Kryo$writeClassAndObject$1.call(Unknown Source)
        at nextflow.util.KryoHelper.serialize(SerializationHelper.groovy:166)
        at nextflow.util.KryoHelper$serialize.call(Unknown Source)
        at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:48)
        at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:113)
        at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:125)
        at nextflow.processor.TaskContext.serialize(TaskContext.groovy:181)

test.nf

#!/usr/bin/env nextflow

import org.yaml.snakeyaml.Yaml

objects = [
  [ firstName:'John', lastName:'Doe' ],
  [ firstName:'Jane', lastName:'Doe' ]
]

process foo {
  input:
  val x from objects

  output:
  val x into receiver

  exec:
  println("foo: '${x.firstName}'")
  
  paramsObj = [:]
  paramsObj['name'] = x.firstName + " " + x.lastName
  paramsFilePath = new File("${task.workDir}/params.yaml")
  yaml = new Yaml()
  yaml.dump(paramsObj, new FileWriter(paramsFilePath))

}

receiver.println { "Received: ${it.firstName}" }