SQL & PL/SQL

Hello.
I have the following procedure. All it does is open a cursor for an SQL string passed into it, and return the open cursor.
PROCEDURE sp_execute_dynamic (hold_input_string IN CLOB,
hold_cursor OUT hold_cursor_type) IS
BEGIN
OPEN hold_cursor FOR TO_CHAR(hold_input_string);
END sp_execute_dynamic;

It works fine except when I perform SELECT DISTINCT. I get the following error.
SQL> declare
2 TYPE hold_cursor_type IS REF CURSOR;
3 hold_cursor hold_cursor_type;
4 hold_object_name VARCHAR2(1024);
5 hold_object_type VARCHAR2(1024);
6 begin
7 dynamic_sql_pkg.sp_execute_dynamic('select distinct object_name from user_objects where object_
name in (''PLAN_TABLE'',''DBA_OBJECTS'')',hold_cursor);
8 loop
9 fetch hold_cursor into hold_object_name, hold_object_type;
10 exit when hold_cursor%NOTFOUND;
11 dbms_output.put_line('Object Name = '||hold_object_name||' Object Type = '||hold_object_type);
12 end loop;
13 end;
14 /
declare
*
ERROR at line 1:
ORA-01007: variable not in select list
ORA-06512: at line 9

It does the same thing with SELECT UNIQUE or SELECT with a GROUP BY. Can anyone tell me why this happens and what I could to to work around it?
Thanks
Chris