OCI bulk insert failure with different NLS_LANG setting

I found a weird problem in OCI bulk insert. It failed if client locale setting
is different from DB server. my DB server locale is NLS_LANG=AMERICAN_AMERICA.ZHT16BIG5 and the table is very simple: SQL> desc foo

 Name                                      Null?    Type
 ----------------------------------------- --------


----------------------------

 A                                                  VARCHAR2(200)
 B                                                  VARCHAR2(200)
 C                                                  NUMBER(9)

If I set client locale same as
server,i.e.NLS_LANG=AMERICAN_AMERICA.ZHT16BIG5, bulk insert OCIStmtPrepare(), OCIBindByPos(), OCIStmtExecute() runs sucessfully.

However, if I set client locale as NLS_LANG=AMERICAN_AMERICA.UTF8, it throws
bind failure:
OCI Error: ORA-01461: can bind a LONG value only for insert into a LONG column

What's more strange is that if the table has only 2 columns as follow, bulk insert now succeeds with any locale setting. Anyone can help me out ? Thanks.

SQL> desc foo

 Name                                      Null?    Type
 ----------------------------------------- --------


----------------------------

 A                                                  VARCHAR2(200)
 C                                                  NUMBER(9)