GET PARAMETER ID not working

Hi,

I am creating a BADI implementation wherein I need to use the purchase order in my logic. I need to use purchase order from EKKO where the field is EBELN. the Parameter ID for EBELN is BES. So i used this in my BADI. Below is the code that I have written to fetch the parameter id in my BADI.

DATA: l_ebeln TYPE ekko-ebeln.
GET PARAMETER ID 'BES' FIELD l_ebeln.

SELECT SINGLE * FROM ekpv INTO wa_ekpv
WHERE ebeln = l_ebeln

However, when i check in debugging, I can see that the field l_ebeln is blank. No value has been fetched. What could be the problem?