ORA-01741: illegal zero-length identifier while create index

  create unique index quia.USERS_U5
on quia.USERS(CASE WHEN (
ORGANIZATION_ID IS NOT NULL
AND STUDENT_ID IS NOT NULL
AND NVL(ACCOUNT_TYPE,0)=2
AND NVL(IS_REMOVED_FROM_MASTER_LIST,'N')='N')
THEN TO_CHAR(ORGANIZATION_ID)||' '||LOWER(STUDENT_ID)
ELSE NULL
END)
tablespace DLNI_TS
online

That's the statement that is getting this error:

ERROR at line 8:
ORA-00604: error occurred at recursive SQL level 1
ORA-01741: illegal zero-length identifier