showing return value of stored procedure in Birt report

Massimo

Hello, i've the following problem:
i've created a function in sql that returns a value. This function takes in input a bound parameter.
I've created the dataset based on this function in Birt. In the query i've put the call to the function:
{? = call db.my_function(?)}

This function seems to be called correctly because "Preview output parameters" showed the correct value.

But when i put the output parameter in Expression Builder and run the report nothing is shown. Note that any error is reported.

What i'm doing wrong?

N.B.: i'm using the Birt version 2.3.0
Thanks in advance
Massimo