mandatory field (without obligatory) - multiple selection

Hello all,

I have a little problem. I have declared a mandatory field (without obligatory).

However, then the multiple selection does not work without having already inserted a material.

My code looks like this right now:

at selection-screen on s_matnr.
if s_matnr[] is initial.

MESSAGE 'The material number must be filled' type 'E'.

leave LIST-PROCESSING.
endif.

Is there any way to work around this so that the multiple selection works when no material is filled?

Thank you.