I want to get claim reason in the script

job = hudson.model.Hudson.instance.getItem("demo2")
build = job.getLastBuild()
println build
def action = build.getAction(hudson.plugins.claim.ClaimBuildAction.class)
print action.getReason()

The script works well under jenkins script console (/script), while when it is executed as global post script, it reports

Failed to execute: claim.groovy, No such property: hudson for class: Script1

in build.getAction(), it seems I can't pass the class name inside.