Problem with sum of variables from 2 subreport

[anhmyphap99]

Hello,

I have a main report and inside two different subreports.

From each of the subreport I send back to the parent report a "returning value" (numerical value defined as java.lang.Double.
Imagine $V{value_1} is the formula for the value returned from the first subreport and  $V{value_2} is the formula for the value returned from the second subreport.
They are processed and passed back fine, infact if I insert $V{value_1} and $V{value_2} in the main report, they show correctly.

My problem is now that I need to define a formula in the main report to make the sum of the 2 values.
I created a variable, $V{totalvalue}, defined as java.lang.Double; the formula is: $V{value_1} + $V{value_2}

If I insert it in the main report $V{totalvalue},  it display always "null".
It is probably due to an issue with the "time of evaluation" but I'm not able to figure out the correct settings I have to use.

Can someone help me?