foreign key constraint not being respected with type created by CREATE TYPE

Hello,

I have created a new data type using CREATE TYPE.
Foreign keys constaints are not being respected with
columns using the new type. As far as I can tell the
new type is working correctly except for foreign keys.
May be I have missed something when defining the new
type. Is there something specific that needs to be
done so that columns using the new type can be used as
foreign keys?
Thanks

CREATE TYPE X (
INTERNALLENGTH = 16,
INPUT = X_in,
OUTPUT = X_out,
RECEIVE = X_recv,
SEND = X_send,
alignment = CHAR
);

CREATE OPERATOR CLASS X_ops
DEFAULT FOR TYPE uuid USING btree
AS
OPERATOR 1 < ,
OPERATOR 2 <= ,
OPERATOR 3 = ,
OPERATOR 4 >= ,
OPERATOR 5 > ,
FUNCTION 1 X_cmp(X, X);

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