PostgreSQL documentation specifies 2-element array for float8_accum but 3-element array expected

I was learning how to create my own aggregate functions from
http://www.postgresql.org/docs/9.0/static/xaggr.html

I copied the avg() example as

CREATE TABLE numbers (
value integer
);

insert into numbers values (2);
insert into numbers values (3);
insert into numbers values (4);

CREATE AGGREGATE myavg(float8) (
sfunc = float8_accum,
stype = float8[],
finalfunc = float8_avg,
initcond = '{0,0}'
);

On trying to run this, I get this error:

cdb=# select myavg(value) from numbers;
ERROR: float8_accum: expected 3-element float8 array

How can I fix this?