GET/SET Parameters not working

Hi

I have a problem in setting and receiving parameter id's

What I want to do is SET a value in a module in the PBO of a screen then get the value to be used in a search help function

What I'm getting is a return code 4 when trying to do the GET

The code I'm using is

**** set parameter in the screen module

MODULE ZSETPARMS OUTPUT..
data l_SUB type subty.
move P0102-SUBTY to l_sub.

SET PARAMETER ID 'ZSUB' Field l_sub

*****The get statement in the search help

IF CALLCONTROL-STEP = 'PRESEL1'.

data l_sub type subty.

data l_selopt type DDSHSELOPT.

get parameter id 'ZSUB' field L_SUB..

The <pid> is registered in TPARA

Could anyone tell me where I'm going wrong

thanks