这个语句有什么有关问题

    Declare     tablename nvarchar2(32);    CURSOR  cur1 is     select to_char(OBJECT_NAME) from all_objects where owner='SYSTEM' and to_char(created,'YYYY-MM-DD')='2011-06-08' and object_type='TABLE';     begin         if cur1%isopen = false then              Open cur1;         end if;                 Loop                                            FETCH cur1 INTO tablename;                                                                        Exit when cur1%NOTFOUND;                                    execute immediate('DROP TABLE '||tablename) ; -- 这里提示无效SQL语句                                                  End Loop;                          Close cur1;  end;