heap_mark4update: (am)invalid tid

Hi,

I am using 7.3.2 on redhat linux 7.3.

I am getting this error during update. My update sql is,

test_pg=# update po_header set emp_name = a.user_name from users a where
emp_code = a.user_code and emp_name != a.user_name and a.user_name is not
null;

I referred the link,
http://archives.postgresql.org/pgsql-bugs/2003-04/msg00076.php .

My answers are,

(a) yes, i have before triggers
(b) no, users table donot have multi entries

Should i require to upgrade? or, Is there any smart sql?

TIA.

regards,
bhuvaneswaran

Table definitions:
                            Table "public.po_header"
       Column        |           Type           |           Modifiers
---------------------+--------------------------+--------------------------------
 po_no               | text                     |
 seq_x               | integer                  |
 seq_y               | integer                  |
 comp_code           | text                     |
 generated_by        | text                     |
 supp_code           | text                     |
 emp_code            | text                     |
....
 emp_name            | text                     |
...
Indexes: po_header_uniq_idx unique btree (po_no, comp_code, seq_x, seq_y),
...
Check constraints: "po_header_type" ((("type" = 'NEW'::text) OR ("type" = 'CHANGED'::text)) OR ("type" =
'CANCELLED'::text))
Triggers: RI_ConstraintTrigger_17175491,
...
          tapproval_date,
          tbuyer_supp_map,
          texception,
          tmtime_po_header,
          tnames_po_header,
          tpo_flags_insert,
          tprocess_complete,
          tref_supplier,
          tstatus_count

                                     Table "public.users"
      Column       |           Type           |                   Modifiers
-------------------+--------------------------+-----------------------------------------------
 user_id           | integer                  | not null default nextval('user_id_seq'::text)
 login_id          | text                     |
 user_code         | text                     |
 comp_code         | text                     |
 user_name         | text                     |
...
Indexes: users_pkey primary key btree (user_id),
...
Check constraints: "users_email_id" ((email_id ~ '@'::text) AND (email_id ~ '.'::text))
Triggers: RI_ConstraintTrigger_17175378,
...
          tmtime_users