Not sure why advancement not working

I don't know what I've done wrong which is causing it to not work as intended, it's meant to give you the advancement if you kill a snow golem and an iron golem with a diamond sword...
{
    "display": 
    {
        "icon":
        {
            "id": "minecraft:iron_ingot"
        },
        "title": "Golem Slayer",
        "description": "Kill both golem types with a diamond sword.",
        "frame": "challenge",
        "background": "minecraft:textures/block/amethyst_block.png"
    },
    "criteria": {
        "killed_iron_golem": {
            "trigger": "minecraft:player_killed_entity",
            "conditions": {
                "entity": {
                    "type": "minecraft:iron_golem",
                    "nbt": "{PlayerCreated:1b}"
                },
                "killing_blow": {
                    "source_entity": {
                        "nbt": "{SelectedItem:{id:"minecraft:diamond_sword"}}"
                    }
                }
            }
        },
        "killed_snow_golem": {
            "trigger": "minecraft:player_killed_entity",
            "conditions": {
                "entity": {
                    "type": "minecraft:snow_golem"
                },
                "killing_blow": {
                    "source_entity": {
                        "nbt": "{SelectedItem:{id:"minecraft:diamond_sword"}}"
                    }
                }
            }
        }
    },
    "requirements": [
        ["killed_iron_golem"],
        ["killed_snow_golem"]
    ],
    "rewards": {
        "recipes": [
            "minecraft:dispenser"
        ],
        "loot": [
            "minecraft:entities/creeper",
            "minecraft:entities/wither"
        ],
        "experience": 10000
    }
}