I created an alarm for vim.event.VmRemovedEvent, but not triggered

I created an alarm monitor the virtual machine is removed.Alarm is created successfully, but can not be triggered. And in the same way to create an alert to monitor the virtual machine is created,it will be triggered.How can I through the alarm to monitor the virtual machine to be deleted?
my code like this:
` service_instance = get_service_instance()
content = service_instance.content
folder = content.rootFolder
expression = vim.alarm.EventAlarmExpression(eventType=eventType, eventTypeId='vim.event.{0}'.format(str(eventType._wsdlName)), objectType=vim.VirtualMachine, status='yellow')

action = vim.alarm.AlarmTriggeringAction(
            action=vim.action.RunScriptAction(script='/bin/sh /opt/alarm.sh'),
            transitionSpecs=[vim.alarm.AlarmTriggeringAction.TransitionSpec(startState='green',finalState='yellow',repeats=False)],
            green2yellow=True, yellow2red=False, red2yellow=False, yellow2green=False)

setting = vim.alarm.AlarmSetting(toleranceRange=0, reportingFrequency=0)

spec = vim.alarm.AlarmSpec(name='alarm_to_smc_{0}'.format(name),
                            description='create alarms by python api: {0}'.format(name),
                            enabled = True, actionFrequency = 0,
                            expression = expression,
                            action = action,
                            setting = setting)

this_alarm = service_instance.content.alarmManager.CreateAlarm(entity=folder, spec=spec)

`

Start at the vim.alarm.EventAlarmExpression setup and the CreateAlarm call shown in the report, then compare the eventType, eventTypeId, and objectType values with the corresponding alert configuration. Reproduce a virtual-machine removal event and inspect whether the created alarm changes state and invokes the configured action; done means the removal event reliably triggers the alarm.