simple query returns an error

In one of our data warehouse database, the following query returns 10,000 rolls;PATIENT_KEY from tdls_clarity_tdl_tran_fact ;

However, this query returns errors:
select PATIENT_KEY from tdls_clarity_tdl_tran_fact where patient_key=702332
                        *
ERROR at line 1:
ORA-00604: error occurred at recursive SQL level 1
ORA-01405: fetched column value is NULL

Anyone know why this happens?

Thanks.
Yinhai