Runout sensor

Hi,
I am getting an error message regarding the filament runout switch - "The value 'my_sensor' is not valid for SENSOR".
The printer is a V-Core 3, BTT Octopus Pro (F429), basic runout switch set up on one of the end stop switch ports (J28), latest version of RatOS.
The printer.cfg setting work on another almost identical printer. I have tried changing the runout switches, I reinstalling RatOS from scratch. I cannot work out why I keep getting this error.

The printer.cfg entry iro the runout switch is:

[filament_switch_sensor RunoutSensor]
pause_on_runout: True
#runout_gcode: M600
switch_pin: PG12 #this relates to J28 on the BTT Octopus Pro

I have tried changing "RunoutSensor" to "my_sensor" and I tried including "runout_gcode: M600" instead of it being commented out but it doesn't help.
No doubt I am missing something so any help would be very much appreciated.
Thanks,
David