Jenkinsfile with validation dsl (does not work!)

Jenkinsfile

validator {
    validate 'hello'
    validate 'world'
}

job output

[Pipeline] node
Running on master in /var/jenkins_home/workspace/validator
[Pipeline] {
[Pipeline] stage
[Pipeline] { (Validate)
[Pipeline] echo
hello
[Pipeline] echo
world
[Pipeline] }
[Pipeline] }
[Pipeline] End of Pipeline
Finished: FAILURE

Validator.groovy

// src/conveyor/Validator.groovy
package conveyor
    
class Validator {
    
    def messages = []
    
    def validate(String msg) {
        messages.add(msg)
    }
}

validator.groovy

// vars/validator.groovy
import conveyor.Validator
    
def call(Closure body) {
    def validator = new Validator()

    body.resolveStrategy = Closure.DELEGATE_FIRST
    body.delegate = validator
    body()

    node('master') {
        stage('Validate') {
            for(def m : validator.messages) {
                echo(m)
            }
        }
    }
}

Why does the pipeline fail?