Advancements Help

ok so i have an issue with a datapack i am working on. basically i have an advancement that is not loading. the file path is:
\data\dragonsummoning\advancements\kill_strato_dragon.json

and the file itself is:
{
  "display": {
    "icon": {
      "item": "minecraft:dragon_head"
    },
    "title": {
      "text": "Stratospherical End",
      "color": "gold"
    },
    "description": {
      "text": "Kill the Stratospherical Shard of the Dragon",
      "color": "yellow"
    },
    "frame": "challenge",
    "show_toast": true,
    "announce_to_chat": true,
    "hidden": false
  },
  "criteria": {
    "requirement": {
      "trigger": "minecraft:player_killed_entity",
      "conditions": {
        "entity": [
          {
            "condition": "minecraft:entity_properties",
            "entity": "this",
            "predicate": {
              "type": "minecraft:ender_dragon",
              "nbt": "CustomName:'{\"text\":\"Stratospherical Shard of the Dragon\"}'"
            }
          }
        ]
      }
    }
  },
  "rewards": {
    "function": "dragonsummoning:leavestrato",
    "loot": [
      "dragonsummoning:strato_dragon_death"
    ]
  }
}

can anyone see any glaring errors that i have made, and that would cause the advancement to not work?