SQL & PL/SQL

hi,

when I run below query, it always throws error below:

SELECT connect_by_root code AS code_root,
    CODE,
    CODE_DESC,
    UP_CODE 
  FROM DI_CODE_LEVEL
  WHERE TYPE_CODE='SP'
    CONNECT BY PRIOR CODE=UP_CODE;

ORA-01436: CONNECT BY loop in user data
01436. 00000 -  "CONNECT BY loop in user data"

But when I use NOCLYCLE and CONNECT_BY_ISCYCLE to find out which row is in question,
it shows nothing, which means CONNECT_BY_ISCYCLE=ZERO for all rows.

select * from (
    SELECT connect_by_root code AS code_root,
        CODE,
        CODE_DESC,
        UP_CODE ,
        CONNECT_BY_ISCYCLE AS ISCYCLE
      FROM DI_CODE_LEVEL
      WHERE TYPE_CODE='SP'
        CONNECT BY NOCYCLE PRIOR CODE=UP_CODE
    ) 
where ISCYCLE>0;

Is there any other reason that this sql would throw ORA-01436?
If there is a loop, then will the combination of NOCYCLE, CONNECT_BY_ISCYCLE will show?

Thanks