Why can't I create two tables that are foreign keys for each other?

Here is my sql script to create two tables that are foreign keys for
each other.

BEGIN;
CREATE TABLE t1 (
id serial PRIMARY KEY,
t2_id integer REFERENCES t2 INITIALLY DEFERRED
);
CREATE TABLE t2 (
id serial PRIMARY KEY,
t1_id integer REFERENCES t1 INITIALLY DEFERRED
);
END;

psql complains the following:
ERROR: Relation 't2' does not exist
ERROR: Relation 't1' does not exist

What's the matter?

Thanks in advance,

--
Eric Du
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