8.1 substring bug?

Consider the following:

CREATE TEMP TABLE tbl (
id SERIAL NOT NULL,
PRIMARY KEY (id)
);

COPY tbl (id) FROM stdin;
1
2
3
4
\.

SELECT substring ('1234567890' FOR (SELECT count (*) FROM tbl)::int);

This returns '1234', as expected. But

SELECT substring ('1234567890' FOR (SELECT count (*) FROM tbl));

returns NULL. I think the problem is that "SELECT count(*)" returns a
BIGINT whereas "substring" expects an INT. Shouldn't there be a warning?