when running the following pipeline script:

stage('1') {
when

}

the job fails with an excehtion:

java.lang.NoSuchMethodError: No such DSL method 'when' found among steps [ansiblePlaybook, archive, bat, build, catchError, checkout, deleteDir, dir, dockerFingerprintFrom, dockerFingerprintRun, echo, emailext, emailextrecipients, envVarsForTool, error, fileExists, getContext, git, input, isUnix, libraryResource, load, mail, milestone, node, parallel, properties, publishHTML, pwd, readFile, readTrusted, resolveScm, retry, script, sh, slackSend, sleep, stage, stash, step, svn, timeout, timestamps, tool, unarchive, unstash, waitUntil, withContext, withCredentials, withDockerContainer, withDockerRegistry, withDockerServer, withEnv, wrap, writeFile, ws] or symbols [all, always, ant, antFromApache, antOutcome, antTarget, any, apiToken, architecture, archiveArtifacts, artifactManager, batchFile, booleanParam, branch, buildButton, buildDiscarder, caseInsensitive, caseSensitive, choice, choiceParam, clock, cloud, command, cron, crumb, defaultView, demand, disableConcurrentBuilds, docker, dockerfile, downloadSettings, downstream, dumb, envVars, environment, expression, file, fileParam, filePath, fingerprint, frameOptions, freeStyle, freeStyleJob, git, hyperlink, hyperlinkToModels, installSource, jdk, jdkInstaller, jgit, jgitapache, jnlp, jobDsl, jobName, junit, label, lastDuration, lastFailure, lastGrantedAuthorities, lastStable, lastSuccess, legacy, list, local, location, logRotator, loggedInUsersCanDoAnything, masterBuild, maven, maven3Mojos, mavenErrors, mavenMojos, mavenWarnings, myView, nodeProperties, nonStoredPasswordParam, none, overrideIndexTriggers, paneStatus, parameters, password, pattern, pipeline-model, pipelineTriggers, plainText, plugin, pollSCM, projectNamingStrategy, proxy, queueItemAuthenticator, quietPeriod, run, runParam, schedule, scmRetryCount, search, security, shell, skipDefaultCheckout, slave, stackTrace, standard, status, string, stringParam, swapSpace, text, textParam, tmpSpace, toolLocation, unsecured, upstream, usernameColonPassword, usernamePassword, viewsTabBar, weather, zfs, zip] or globals [currentBuild, docker, env, manager, params, pipeline, scm]