MongoDB update array appears can ' t append to array using string field name

The database content is probably as follows:

{_id:1,"hero_list" : {                "15521" : {                        "stars":0,                        "_id":15521,                        "Equip" : [                                [ ],                                [ ],                                [ ], {                                        "item_id":310141,                                        "LV":103,                                        "UUID":"4a727ee1-e7b0-4265-b004-e2b75890378a",                                        "Amount":1                                },                                [ ],                                [ ] ],                        "LV": the,                        "Exp":0,                        "Quality":0                }            }}

> db.hero.update ({_id:1},{$set: {"hero_list.15521.equip.4.lv": 1}})
Can ' t append to array using string field name [LV]

For a long while, the original is their own eyesight, the array of subscript number is wrong

Db.hero.update ({_id:1},{$set: {"hero_list.15521.equip.3.lv": 1}})

Ok

MongoDB update array appears can ' t append to array using string field name