Query failing with strange error.

In the middle of a script, inside a transaction, I run the following
query:

update customer_contact
 set next_billed=min(customer_services.eff_date)
 where customer_contact.conn_num=7698
  and customer_services.conn_num=7698
  and customer_services.inv_num=0

I have also tried changing this to

update customer_contact
 set next_billed=min(customer_services.eff_date)
 where customer_contact.conn_num=7698
  and customer_services.conn_num=customer_contact.conn_num
  and customer_services.inv_num=0

... which changes the query plan, but both give the following error:

ERROR:  ExecutePlan: (junk) `ctid' is NULL!

Now... I tried dumping and reloading the database.  Then I upgraded
from 7.2.1 to 7.2.3 (and reloaded the database from scratch).  Neither
worked.

The odd part about the error is that this query gets executed 100's of
times a day with different conn_num values.  Only this conn_num gives
the error.  This conn_num exists in both tables.  eff_date has a
senable value (it's a 'timestamp with timezone').  In fact:

sales=# select min(next_billed),min(customer_services.eff_date)
     from customer_contact, customer_services
     where customer_contact.conn_num=7698
      and customer_services.conn_num=7698
      and customer_services.inv_num=0;

    min     |          min
------------+------------------------
 2003-01-23 | 2003-01-23 00:00:00-05
(1 row)

Anyone got any ideas?

Dave.

--
============================================================================
|David Gilbert, Velocet Communications.       | Two things can only be     |
|Mail:       dgilbert@velocet.net             |  equal if and only if they |
|http://daveg.ca                              |   are precisely opposite.  |
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