count selected rows for a group

Hi,

I want to show the number of selected rows of a group (tree level) in a treeview datawindow (PB 12.1 classic).

I have created a computed field in the tree level header with

     sum (if (IsSelected(), 1, 0) for group 1)

but it always shows 0.

I've tested it with a computed field for each row (expression: if (IsSelected(), 1, 0) ). This result is as expected 0 or 1. If I sum this I get 0.

I've tested it with a field in trailer band too but the result is always 0.

But if I sum it for all I get the expected result.

How can I get the number of selected rows for a single group?