Additional_fields dropdown not updating with api

I have an additional field, the details of which can be seen below for an existing user

{
        "id": "4",
        "title": "Office Location",
        "type": "dropdown",
        "mandatory": true,
        "invisible_to_user": false,
        "settings": null,
        "sequence": 4,
        "options": [
          {
            "id": "1",
            "label": "CRICK"
          },
          {
            "id": "3",
            "label": "Edinburgh"
          },
          {
            "id": "13",
            "label": "Lynton House"
          },
          {
            "id": "2",
            "label": "Stevenage"
          }
        ],
        "value": "13",
        "enabled": true
      }

When I try and use the /manage/v1/user PUT endpoint with the following value in the additional_fields for a different user it just returns a 500

{"4","13"}

The same happens if I use “1”, “3” or “2” as well for the value

Any ideal what I am doing wrong?