Exec binding: deal with missing values

I am using two raspberrys, one for running openHAB, the other one sits in my electronics rack and is running kodi and at the same time has a DS18B20 temperature sensor connected to measure the temperature in the rack. I read the temperature via an exec binding (ssh login from openHAB machine to kodi machine and then reading the sensor output file) which works fine.
My problem: the kodi machine is not constantly running. During the time when the machine is down I would like to set the rack temperature to “undefined” (i.e no temperature displayed on the openhab Number Item and a gap in the chart showing the temperature). So far openHAB retains the last value received from the kodi machine after the kodi machine goes down and keeps this value all the way until the machine comes up again. I have tried to set the return value of the exec binding script to “NaN” when the ssh login fails, but without success. Any value that cannot be interpreted as a number seems to make openHAB retain the last valid value until a new valid numeric value is received.
Is there any way to set a Number item with an exec binding to “undefined” or “uninitialized” (like the state when openHAB is started and no value has yet been received).

I’m digging way back into my memory but I don’t think you can set an Item to Undefined. It is a special state that only OH can set an Item to. I’m also pretty sure that any binding will treat anything that results in a NaN as an error.

The best advice I have is to set it to some absurd number like -1.