Aggregate Function (AVG) not calculated correctly

Gavin Evans (gavin(at)consultant(dot)com) reports a bug with a severity of 1
The lower the number the more severe it is.

Short Description
Aggregate Function (AVG) not calculated correctly

Long Description
Using the following code:
SELECT airline_code, AVG(distance) FROM flights GROUP BY airline_code;

The first line of the result set (AI | -2864) is calculated wrongly.

I have included all code needed to prove the bug, this is a SERIOUS BUG!

G

Sample Code
eg3ab=# select * from flights;
flight_number | aircraft | distance | airline_code | origin | destination
---------------+----------+----------+--------------+--------+-------------
AI434 | 310 | 3417 | AI | BOM | PER
AI306 | 747 | 1724 | AI | BOM | BKK
AI410 | 310 | 2012 | AI | BOM | KUL
AI158 | 740 | 2720 | AI | FRA | BOM
AI112 | 744 | 9632 | AI | JFK | HKG
CX254 | 744 | 8736 | CX | LHR | HKG
VS200 | 340 | 8736 | VS | LHR | HKG
VS501 | 744 | 14737 | VS | LHR | SYD
BA009 | 744 | 12882 | BA | LHR | SYD
QF10 | AB3 | 13368 | QF | LHR | SYD
GA881 | M11 | 8035 | GA | LGW | BKK
PR731 | 74E | 9148 | PR | LGW | BKK
TG915 | 744 | 10905 | TG | LHR | BKK
TG911 | 744 | 9422 | TG | LHR | SIN
BA011 | 744 | 9422 | BA | LHR | SIN
AI180 | 747 | 6422 | AI | LHR | SIN
QF12 | AB3 | 1839 | QF | MEL | PER
BA014 | 767 | 321 | BA | LHR | FRA
AI200 | 747 | 3705 | AI | JFK | FRA
AI181 | 747 | 6422 | AI | SIN | LHR
AI201 | 747 | 3705 | AI | FRA | JFK
(21 rows)

eg3ab=# \i ex13.query
airline_code | avg
--------------+-------
AI | -2864
BA | 7541
CX | 8736
GA | 8035
PR | 9148
QF | 7603
TG | 10163
VS | 11736
(8 rows)

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