Failed to parse saved optimizer context: error reading ranges value

set default_storage_engine=myisam;

CREATE TABLE t1(a VARCHAR(128),b VARCHAR(128),KEY(A),KEY(B));

INSERT INTO t1 SELECT REPEAT('a',128),REPEAT('b',128) FROM seq_1_to_64;

SET SESSION sort_buffer_size=1024*8;

set optimizer_record_context=1;

EXPLAIN 

SELECT * FROM t1 FORCE INDEX(a,b) WHERE a LIKE 'a%' OR b LIKE 'b%' 

ORDER BY a,b;

select context into dumpfile '/tmp/1.sql' from information_schema.optimizer_context;

source /tmp/1.sql

+------+-------------+-------+-------------+---------------+------+---------+------+------+----------------------------------------------------+

| id   | select_type | table | type        | possible_keys | key  | key_len | ref  | rows | Extra                                              |

+------+-------------+-------+-------------+---------------+------+---------+------+------+----------------------------------------------------+

|    1 | SIMPLE      | t1    | index_merge | a,b           | a,b  | 515,515 | NULL | 64   | Using sort_union(a,b); Using where; Using filesort |

+------+-------------+-------+-------------+---------------+------+---------+------+------+----------------------------------------------------+

+------+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+

| id   | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                                               |

+------+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+

|    1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Impossible WHERE noticed after reading const tables |

+------+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+

1 row in set, 1 warning (0.001 sec)

Warning (Code 4253): Failed to parse saved optimizer context: error reading ranges value at offset 465.

Query OK, 0 rows affected (0.000 sec)