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!