Can't register python class for domain?

I created this domain:

create domain milliamp as double precision check (value >= 0);

Then when I register a python class named Milliamp, it never gets
used! I think the problem is related to the type code coming back as
the underlying double precision type:

>>> cursor.execute("select 4444::milliamp as m")
>>> cursor.description[0][1]
701
>>> cursor.execute("select 4444::double precision as f")
>>> cursor.description[0][1]
701

But there really is a type called milliamp in postgresql, I promise:

> select typname, oid from pg_type where typname in ('float8', 'milliamp');
typname oid
-------- ------
float8 701
milliamp 949531
(2 rows)

I found this code, and it suggests other people have run into this same problem:

https://github.com/beyang/psycopg2/blob/master/sandbox/domainoid.py

Any ideas what is going on?

Thanks in advance. I love psycopg!