Select Options Error :Entered Selections were not accepted

Dear All,

I have used the following code to populate select-options when the Report runs.But when i click Multiple Selection and come back to

selection screen it displays the below message.Please advise.

Code:

SELECT-OPTIONS s_unam for sy-uname.

INITIALIZATION.

s_unam-sign = 'I'.

s_unam-OPTION = 'BT'.

s_unam-low = 'ZA'.

s_unam-high = 'ZZ'.

APPEND s_unam.

Thanks

Prabaharan