ORA-01407: cannot update ("IUM"."TELSXTOWNCODE"."TOWNCODE") to NULL

Hi experts.

I have the following problem, if you can help.

ORA-01407: cannot update ("IUM"."TELSXTOWNCODE"."TOWNCODE") to NULL

This is the script:

update ium.TELSXTOWNCODE t

set t.TOWNCODE = (select TOWNCODE from test t2 where t.dn = t2.dn)

test table:

Connected to:

Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production

With the Partitioning, Real Application Clusters, Automatic Storage Management, OLAP,

Data Mining and Real Application Testing options

SQL> desc test

Name                                      Null?    Type

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

DN                                        NOT NULL VARCHAR2(10)

TOWNCODE                                  NOT NULL VARCHAR2(5)

STATUS                                             VARCHAR2(5)

desc  ium.TELSXTOWNCODE

SQL> desc  ium.TELSXTOWNCODE

Name                                      Null?    Type

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