Wrong result upon query using index_merge with DESC key

--source include/have_innodb.inc

CREATE OR REPLACE TABLE t (pk INT, a INT, b int, KEY(a), PRIMARY KEY(pk DESC)) ENGINE=InnoDB;

INSERT INTO t VALUES (1,4,5),(2,9,6),(3,NULL,7),(4,NULL,8);

ANALYZE TABLE t PERSISTENT FOR ALL; # Optional, fails either way

SELECT * FROM t WHERE pk > 10 OR a > 0;

# Cleanup

DROP TABLE t;

SELECT * FROM t WHERE pk > 10 OR a > 0;

pk	a	b

EXPLAIN

{

  "query_block": {

    "select_id": 1,

    "nested_loop": [

      {

        "table": {

          "table_name": "t",

          "access_type": "index_merge",

          "possible_keys": ["PRIMARY", "a"],

          "key_length": "5,4",

          "index_merge": {

            "sort_union": [

              {

                "range": {

                  "key": "a",

                  "used_key_parts": ["a"]

                }

              },

              {

                "range": {

                  "key": "PRIMARY",

                  "used_key_parts": []

                }

              }

            ]

          },

          "rows": 3,

          "filtered": 100,

          "attached_condition": "t.pk > 10 or t.a > 0"

        }

      }

    ]

  }

}