{host:vfs.fs.size[/,pfree].last(0)}<30 do not trigger event/email

[stevenfoo]

I have configure the following and it did not generate any event or email alert. Only trigger is ON.

{host:vfs.fs.size[/,pfree].last(0)}<30

However when I use the setting below for bytes size, it will generate event and send email. Trigger is also ON.

{host:vfs.fs.size[/,free].last(0)}<bytes size

Any ideas why pfree does not generate event and email?

I am running zabbix on 1.4.4 on Linux/Mysql/Apache/PHP.

Any help is appreciated.