How to get the active tab in a Tabstrip

Hi experts,

I have a report included a dynpro (1001). On this dynpro I have a tabstrip (TAB_ALV) with 4 tabs (dynpro 1010, 1020, 1030 and 1040).

PBO for 1001:

CALL SUBSCREEN: sub_button INCLUDING sy-repid '1010',
sub_layout INCLUDING sy-repid '1020',
sub_spalte INCLUDING sy-repid '1030',
sub_beispiel INCLUDING sy-repid '1040'.

Okay, everything is working. I can change tabs...

Now I have added in my TOP-include

CONTROLS tab_alv TYPE TABSTRIP.

because I want to know which tab is selected.

But when I try to debug my report (Dialog transaction with dynrpo 1001), the variant TAB_ALV is not assigned...

What am I doing wrong?