Return value of subreport is null

Hi everybody,

I want to use the return value of a subreport into the master report, but the only thing that is shown is "null".  

I'd done the following:

Subreport

create variable "var_subreport" of type BigDecimal, Expression 2005, ResetType= None, IncrementType = None

Mainreport:
create variable "var_mainreport" of type BigDecimal, ResetType = None, IncrementType = None

In Mainreport in the subreport element I set up the following:

From variable: var_subreport
To Variable: var_mainreport
Calculation Type: nothing

The evaluation time of the text field that displayes the return value is set to Report.

I also want to use the value vor a subtraction. So the return value should be substracted from another value. But this also gives null.

Any hint to solve this is appreciated!