The selection screen cannot choose the multiple values at first instance?

Hi All,

I have 2 select-options on the selection screen as follows>>

SELECT-OPTIONS : 
                   s_subusr FOR pa0105-usrid NO INTERVALS OBLIGATORY,    
                  s_period FOR vbak-erdat   NO-EXTENSION OBLIGATORY.    

When I try to enter multiple values by clicking the multiple option button when noting is entered as input on the selection screen.I get a error"Make entry in all required fields".

But when I enter a value on the selection screen input for s_subusr and then enter value in s_period and then click on multiple option button I am able to see the multi-option Popup.Now I can enter multiple Values.

Please can anyone help me to enter the values on the multiple option Pop-up whenever the user want?

Regards,

Shashank.