If I try and debug any function, the debugger always says nvarchar2 values are null.
The sample test script below confirms this - just add v_var and v_nvar to the watch list. v_nvar will always show as null.
Tested using 10.0.5 and 11.0.0.1753 (beta) and get the same results.
declare
v_nvar nvarchar2(10);
v_var varchar2(10);
begin
select '123' into v_var from dual;
select '123' into v_nvar from dual;
select v_var into v_nvar from dual;
end;