Error

I got an error when accessing a view. The view looks like this:

CREATE TABLE ar_contacts(
    contact_seq         int,
    update_date         date,
    period              date,
    paid                float,
    sales               float,
    last_date           date,
    due                 float,
    PRIMARY KEY (contact_seq, period)
);
CREATE VIEW ar_sales90_v AS
    SELECT contact_seq, SUM(sales) AS sales90 FROM ar_contacts WHERE
    period >= date(date_trunc('month','now'::date) - '2 month'::timespan)
    GROUP BY contact_seq;

If I
SELECT * FROM ar_sales90_v WHERE contact_seq = 1;
then everything is fine. I get what I expect:

contact_seq|sales90
-----------+-------
          1|      6
(1 row)

If I do this:
SELECT sales90 FROM ar_sales90_v WHERE contact_seq = 1;
I get this result:

pqReadData() -- backend closed the channel unexpectedly.
This probably means the backend terminated abnormally before or while
processing the request. We have lost the connection to the backend, so
further processing is impossible.  Terminating.

The backend didn't die; I can restart psql and do my queries.