PLEG Error Message ": attempt to compare nil with number"

Developer's Corner Scene Scripting
#1

trigger HWBoost Whenever HW Booster is turned on
device property HWDegC CurrentTemperature[urn:upnp-org:serviceId:TemperatureSensor1]
condition DHWSetPoint (mHome ? (HWBoost ? 70 : (ItsEMorning ? 70 : 63):15)) null
condition DHWHtgOn HWDegC < (DHWSetPoint -10) null
condition DHWHtgOff HWDegC >= DHWSetPoint null
This is my programming to control hot water heater.
The format of the setpoint condition is the same a three others associated with heating zones: they work perfectly.
But this one brings up the error message" HWDegC >= DHWSetpoint: attempt to compare nil with number"
and leaves on and off conditions as ‘nulls’ because it is itself a ‘null’
After hours of trying many variations I cannot see what’s wrong: can anyone solve the problem?

#2

You may want to post this in the PLEG section of the forum. If you do, post a status report. Many folks there can look over the status report and perhaps suggest help.