Subquery returning NULL, why?

[Alan Lee]

I don't understand why the following query will return NULL in column v.

DROP TABLE IF EXISTS tbl_test;
CREATE TABLE tbl_test ( a INT, b INT );
INSERT INTO tbl_test VALUES (1,1),(2,2),(3,3);

SELECT t2.a,
( SELECT t3.a
FROM tbl_test t3
LEFT JOIN tbl_test t4 ON t4.b = 999
WHERE t3.a = t2.a ) AS v
FROM tbl_test t1
JOIN tbl_test t2 ON t2.a = t1.a
WHERE t1.b = 3
GROUP BY t2.a

If I change 999 to 1, the correct value (i.e. 3) is retrieved.
BTW, MySQL version is 5.1.57.