I am unable to use "when" inside a failure block:

#!groovy
pipeline {
  agent { label 'ci-runner' }
  parameters {
    choice(name: 'NOTIFY_SLACK', choices: 'true\nfalse', description: 'Send Slack notification on failure')
  }
  stages {
    stage('Test') {
      steps {
        sh 'exit 1'
      }
    }
  }
  post {
    failure {
      when {
        expression {
          params.NOTIFY_SLACK == 'true'
        }
        slackSend(
          color: "Danger",
          message: "Jenkins job ${env.JOB_NAME} (${env.BUILD_NUMBER}) failed: ${env.BUILD_URL}",
          channel: "#eng-somewhere"
        )
      }
    }
  }
}

The error I get is:

java.lang.NoSuchMethodError: No such DSL method 'when' found among steps [VersionNumber, archive, bat, build, catchError, checkout, deleteDir, dir, dockerFingerprintFrom, dockerFingerprintRun, echo, emailext, emailextrecipients, envVarsForTool, error, fileExists, findFiles, getContext, git, input, isUnix, jiraComment, jiraIssueSelector, jiraSearch, junit, library, libraryResource, load, lock, mail, milestone, node, nodesByLabel, openstackMachine, parallel, powershell, properties, publishHTML, pwd, readFile, readJSON, readManifest, readMavenPom, readProperties, readTrusted, readYaml, resolveScm, retry, script, setGerritReview, sh, sha1, slackSend, sleep, sshagent, stage, stash, step, tee, timeout, timestamps, tm, tool, touch, triggerRemoteJob, unarchive, unstash, unzip, validateDeclarativePipeline, waitUntil, withContext, withCredentials, withDockerContainer, withDockerRegistry, withDockerServer, withEnv, withMaven, wrap, writeFile, writeJSON, writeMavenPom, writeYaml, ws, zip] or symbols [CredentialsAuth, NoneAuth, NullAuth, TokenAuth, agent, all, allOf, always, ant, antFromApache, antOutcome, antTarget, any, anyOf, apiToken, architecture, archiveArtifacts, artifactManager, artifactsPublisher, authorizationMatrix, batchFile, bitbucket, booleanParam, branch, brokenBuildSuspects, brokenTestsSuspects, buildButton, buildDiscarder, buildParameter, buildingTag, caseInsensitive, caseSensitive, certificate, changeAbandoned, changeMerged, changeRequest, changeRestored, changelog, changeset, checkoutToSubdirectory, choice, choiceParam, cleanWs, clock, cloud, command, commentAdded, commentAddedContains, concordionPublisher, configFile, configFileProvider, copyArtifacts, credentials, cron, crumb, culprits, defaultView, demand, dependenciesFingerprintPublisher, developers, disableConcurrentBuilds, disableResume, docker, dockerCert, dockerfile, downloadSettings, downstream, draftPublished, dumb, durabilityHint, envVars, environment, equals, expression, file, fileParam, filePath, findbugsPublisher, fingerprint, frameOptions, freeStyle, freeStyleJob, fromScm, fromSource, ftpPublisher, gerrit, git, github, githubPush, headRegexFilter, headWildcardFilter, hyperlink, hyperlinkToModels, inheriting, inheritingGlobal, installSource, invokerPublisher, isRestartedRun, jdk, jdkInstaller, jgit, jgitapache, jgivenPublisher, jnlp, jobDsl, jobName, junitPublisher, label, lastCompleted, lastDuration, lastFailure, lastGrantedAuthorities, lastStable, lastSuccess, lastSuccessful, latestSavedBuild, legacy, legacySCM, list, local, location, logRotator, loggedInUsersCanDoAnything, masterBuild, maven, maven3Mojos, mavenErrors, mavenLinkerPublisher, mavenMojos, mavenWarnings, modernSCM, myView, newContainerPerStage, node, nodeProperties, nonInheriting, nonStoredPasswordParam, none, not, openTasksPublisher, overrideIndexTriggers, paneStatus, parameters, password, patchsetCreated, pattern, permalink, pipeline-model, pipelineGraphPublisher, pipelineMaven, pipelineTriggers, plainText, plugin, pollSCM, preserveStashes, projectNamingStrategy, proxy, queueItemAuthenticator, quietPeriod, recipients, refUpdated, remotingCLI, requestor, run, runParam, schedule, scmRetryCount, scriptApprovalLink, search, security, shell, skipDefaultCheckout, skipStagesAfterUnstable, slave, snapshotDependencies, sourceRegexFilter, sourceWildcardFilter, specific, sshUserPrivateKey, stackTrace, standard, status, string, stringParam, swapSpace, tag, text, textParam, tmpSpace, toolLocation, topicChanged, unsecured, upstream, upstreamDevelopers, usernameColonPassword, usernamePassword, viewsTabBar, weather, withAnt, workspace, zfs, zip] or globals [currentBuild, docker, env, params, pipeline, scm] at org.jenkinsci.plugins.workflow.cps.DSL.invokeMethod(DSL.java:176) at org.jenkinsci.plugins.workflow.cps.CpsScript.invokeMethod(CpsScript.java:122) at sun.reflect.GeneratedMethodAccessor15831.invoke(Unknown Source) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at org.codehaus.groovy.reflection.CachedMethod.invoke(CachedMethod.java:93) at groovy.lang.MetaMethod.doMethodInvoke(MetaMethod.java:325) at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:1213) at groovy.lang.MetaClassImpl.invokeMethod(MetaClassImpl.java:1022) at org.codehaus.groovy.runtime.callsite.PogoMetaClassSite.call(PogoMetaClassSite.java:42) at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:48) at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:113) at org.kohsuke.groovy.sandbox.impl.Checker$1.call(Checker.java:157) at org.kohsuke.groovy.sandbox.GroovyInterceptor.onMethodCall(GroovyInterceptor.java:23) at org.jenkinsci.plugins.scriptsecurity.sandbox.groovy.SandboxInterceptor.onMethodCall(SandboxInterceptor.java:133) at org.kohsuke.groovy.sandbox.impl.Checker$1.call(Checker.java:155) at org.kohsuke.groovy.sandbox.impl.Checker.checkedCall(Checker.java:159) at org.kohsuke.groovy.sandbox.impl.Checker.checkedCall(Checker.java:129) at com.cloudbees.groovy.cps.sandbox.SandboxInvoker.methodCall(SandboxInvoker.java:17) at WorkflowScript.run(WorkflowScript:16) at org.jenkinsci.plugins.pipeline.modeldefinition.ModelInterpreter.delegateAndExecute(jar:file:/var/lib/jenkins/plugins/pipeline-model-definition/WEB-INF/lib/pipeline-model-definition.jar!/org/jenkinsci/plugins/pipeline/modeldefinition/ModelInterpreter.groovy:133) at org.jenkinsci.plugins.pipeline.modeldefinition.ModelInterpreter.runPostConditions(jar:file:/var/lib/jenkins/plugins/pipeline-model-definition/WEB-INF/lib/pipeline-model-definition.jar!/org/jenkinsci/plugins/pipeline/modeldefinition/ModelInterpreter.groovy:732) at org.jenkinsci.plugins.pipeline.modeldefinition.ModelInterpreter.catchRequiredContextForNode(jar:file:/var/lib/jenkins/plugins/pipeline-model-definition/WEB-INF/lib/pipeline-model-definition.jar!/org/jenkinsci/plugins/pipeline/modeldefinition/ModelInterpreter.groovy:380) at org.jenkinsci.plugins.pipeline.modeldefinition.ModelInterpreter.catchRequiredContextForNode(jar:file:/var/lib/jenkins/plugins/pipeline-model-definition/WEB-INF/lib/pipeline-model-definition.jar!/org/jenkinsci/plugins/pipeline/modeldefinition/ModelInterpreter.groovy:378) at org.jenkinsci.plugins.pipeline.modeldefinition.ModelInterpreter.runPostConditions(jar:file:/var/lib/jenkins/plugins/pipeline-model-definition/WEB-INF/lib/pipeline-model-definition.jar!/org/jenkinsci/plugins/pipeline/modeldefinition/ModelInterpreter.groovy:731) at com.cloudbees.groovy.cps.CpsDefaultGroovyMethods.each(CpsDefaultGroovyMethods:2030) at com.cloudbees.groovy.cps.CpsDefaultGroovyMethods.each(CpsDefaultGroovyMethods:2015) at com.cloudbees.groovy.cps.CpsDefaultGroovyMethods.each(CpsDefaultGroovyMethods:2056) at org.jenkinsci.plugins.pipeline.modeldefinition.ModelInterpreter.runPostConditions(jar:file:/var/lib/jenkins/plugins/pipeline-model-definition/WEB-INF/lib/pipeline-model-definition.jar!/org/jenkinsci/plugins/pipeline/modeldefinition/ModelInterpreter.groovy:727) at org.jenkinsci.plugins.pipeline.modeldefinition.ModelInterpreter.executePostBuild(jar:file:/var/lib/jenkins/plugins/pipeline-model-definition/WEB-INF/lib/pipeline-model-definition.jar!/org/jenkinsci/plugins/pipeline/modeldefinition/ModelInterpreter.groovy:706) at __cps.transform__(Native Method) at com.cloudbees.groovy.cps.impl.ContinuationGroup.methodCall(ContinuationGroup.java:57) at com.cloudbees.groovy.cps.impl.FunctionCallBlock$ContinuationImpl.dispatchOrArg(FunctionCallBlock.java:109) at com.cloudbees.groovy.cps.impl.FunctionCallBlock$ContinuationImpl.fixArg(FunctionCallBlock.java:82) at sun.reflect.GeneratedMethodAccessor381.invoke(Unknown Source) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at com.cloudbees.groovy.cps.impl.ContinuationPtr$ContinuationImpl.receive(ContinuationPtr.java:72) at com.cloudbees.groovy.cps.impl.ClosureBlock.eval(ClosureBlock.java:46) at com.cloudbees.groovy.cps.Next.step(Next.java:83) at com.cloudbees.groovy.cps.Continuable$1.call(Continuable.java:174) at com.cloudbees.groovy.cps.Continuable$1.call(Continuable.java:163) at org.codehaus.groovy.runtime.GroovyCategorySupport$ThreadCategoryInfo.use(GroovyCategorySupport.java:122) at org.codehaus.groovy.runtime.GroovyCategorySupport.use(GroovyCategorySupport.java:261) at com.cloudbees.groovy.cps.Continuable.run0(Continuable.java:163) at org.jenkinsci.plugins.workflow.cps.SandboxContinuable.access$101(SandboxContinuable.java:34) at org.jenkinsci.plugins.workflow.cps.SandboxContinuable.lambda$run0$0(SandboxContinuable.java:59) at org.jenkinsci.plugins.scriptsecurity.sandbox.groovy.GroovySandbox.runInSandbox(GroovySandbox.java:108) at org.jenkinsci.plugins.workflow.cps.SandboxContinuable.run0(SandboxContinuable.java:58) at org.jenkinsci.plugins.workflow.cps.CpsThread.runNextChunk(CpsThread.java:174) at org.jenkinsci.plugins.workflow.cps.CpsThreadGroup.run(CpsThreadGroup.java:332) at org.jenkinsci.plugins.workflow.cps.CpsThreadGroup.access$200(CpsThreadGroup.java:83) at org.jenkinsci.plugins.workflow.cps.CpsThreadGroup$2.call(CpsThreadGroup.java:244) at org.jenkinsci.plugins.workflow.cps.CpsThreadGroup$2.call(CpsThreadGroup.java:232) at org.jenkinsci.plugins.workflow.cps.CpsVmExecutorService$2.call(CpsVmExecutorService.java:64) at java.util.concurrent.FutureTask.run(FutureTask.java:266) at hudson.remoting.SingleLaneExecutorService$1.run(SingleLaneExecutorService.java:131) at jenkins.util.ContextResettingExecutorService$1.run(ContextResettingExecutorService.java:28) at jenkins.security.ImpersonatingExecutorService$1.run(ImpersonatingExecutorService.java:59) at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511) at java.util.concurrent.FutureTask.run(FutureTask.java:266) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748)