Munin - how to return unknown value from plugin

I want my munin plugin to report unknown values if the value can't be reported. What do I have to return?

• "nan"
• "-nan"
• nothing at all, just skip returning a value
• "unknown"
• ...?

Nothing works for me.

Background: ntp_offset plugin fails if ntp is not running at all. I want to return unknown value and send an alert message.

I think the correct value to return is "U".

This is briefly mentioned without explanation in the documentation:

https://munin.readthedocs.io/en/latest/reference/plugin.html#fieldname-value