Oracle - 功能不工作

So i've written a function that compares the logged in student's username, which in this case is the student_no, to the list of students and makes sure that the logged in user is a student. Then it compares the student_no with the enrol table to seek out any programs the user is enrolled into. So that in essence when I SELECT * FROM yaser.enrol (the table owner is yaser) I should see only a few records from the enrol list. Instead I get the error: failed to execute policy function.

这是我的功能:

-- Create policy function to be called when 'ENROL' table is accessed
create or replace function f_policy_enrol (schema in varchar2, tab in varchar2)
-- Function will return a string that is used as a WHERE clause                       
return varchar2
as
 v_student_no     varchar2(10);
 is_student       number:=0;
 v_user           varchar2(100);
 out_string       varchar2(400) default '1=2 ';

begin
  -- get session user
  v_user := lower(sys_context('userenv','session_user'));

  -- Is the user a student?
  begin
    select student_no into v_student_no from student where lower(student_no) = v_user;
    is_student:=1;
  exception
    when no_data_found then
    v_student_no := 0;
  end;

  -- If it's a student, then they are only allowed to see their record only.
  if is_student = 1 then
     out_string := out_string||'or student_no = '||v_student_no||' ';
  end if;

  return out_string;
end;
/

这是我调用的策略:

begin
dbms_rls.add_policy('yaser',
                    'enrol',
                    'accesscontrol_enrol',
                    'yaser',
                    'f_policy_enrol',
                    policy_type => dbms_rls.context_sensitive);
end;
/

如前所述..我不确定我哪里出错了.无论是在政策上还是在功能上.任何帮助将不胜感激！如果您有任何问题，我很乐意在我有空工作时尽快回答！

1=2 or student_no = jcave

out_string := out_string||'or student_no = '''||v_student_no||''' ';