error caused by FOREIGN KEY on composite type

I have the following definitions:

--------------------------------------

create type mytype as (x integer, y integer);

create table foo(
    a mytype primary key,
    b integer
);

create table bar(
    a mytype references foo
);

insert into foo values((0,0)::mytype,0);

--------------------------------------

When I try to do a simple update on foo, I get an error:

test=> update foo set b=1;
ERROR:  no conversion function from mytype to record

Can someone explain what does this error mean?