Oracle Forms

I got this error message in the form9i for the list item deptno that I created. The list item uses the record group called DEPT which lists dname in the popup list and returns deptno to the list item in the data block called EMP.

The code for the DEPT record group is:
<
select dname, to_char(deptno) num
from dept
>
I also created a WHEN-NEW-FORM-INSTANCE trigger at the FORM level. The code for this trigger is:
<
declare

return_value number;

begin

-- populate the list item
return_value := populate_group('DEPT');
populate_list('EMP.DEPTNO', 'DEPT');

-- set the list item to the first sample_id
:EMP.DEPTNO := get_group_char_cell('DEPT', 1);


end;
>

I can see and select the poplist list. There seems nothing wrong with list. But, why did I get this error message? Could anyone help? Thanks!