Incorrect result when subquery has SUM(outer_select_field)

CREATE TABLE t1 (a INT PRIMARY KEY, b INT);

INSERT INTO t1 VALUES (1,1), (2,2);

CREATE TABLE t2 (a INT PRIMARY KEY, b INT);

INSERT INTO t2 VALUES (1,1), (3,3);

set join_cache_level=4;

SELECT SQL_NO_CACHE 

  (SELECT SUM(c.a) FROM t1 ttt, t2 ccc 

   WHERE ttt.a = ccc.b AND ttt.a = t.a GROUP BY ttt.a) AS minid   

FROM t1 t, t2 c WHERE t.a = c.b;

 minid

 NULL

set join_cache_level= 2;

SELECT SQL_NO_CACHE 

  (SELECT SUM(c.a) FROM t1 ttt, t2 ccc 

   WHERE ttt.a = ccc.b AND ttt.a = t.a GROUP BY ttt.a) AS minid   

FROM t1 t, t2 c WHERE t.a = c.b;

minid

1

DROP TABLE t1,t2;