ireport 5.6.0 do calculative between subreport value and main report value

[heeyinhow1988]

Hi all, I am facing a issue while doing calculation within 2 value, 1st value is main report variable, and 2nd value is sub report returned value.

my preview as below:

blue is main report variable, green is subreport variable.

although i have successfully return the value from subreport, but it still unable to do calculation.

this is the formular inside the null : $V{pubunit}.subtract($V{MeterGrandTotal})

 

thanks,

yinhow.