I'm getting a 422 Unprocessable Entity when trying to create a droplet via postman.

First, a GET to https://api.digitalocean.com/v2/images gives me a list of droplet images. Let's say I want to base my droplet on this one...

{
      "id": 13764801,
      "name": "Elixir on 14.04",
      "distribution": "Ubuntu",
      "slug": null,
      "public": true,
      "regions": [
        "nyc1",
        "sfo1",
        "nyc2",
        "ams2",
        "sgp1",
        "lon1",
        "nyc3",
        "ams3",
        "fra1",
        "tor1",
        "sfo2",
        "blr1"
      ],
      "created_at": "2015-09-30T15:42:07Z",
      "min_disk_size": 30,
      "type": "snapshot",
      "size_gigabytes": 1.85
    },

So I POST to https://api.digitalocean.com/v2/droplets with this request body:

{
 "name" : "512-auto",
 "region" : "lon1",
 "size" : "512mb",
 "image" : "Elixir on 14.04",
 "ssh_keys" : ["a9:a2:b4:d2:4f:79:32:22:9d:e3:68:72:11:11:aa:aa"]
}

I get a 422 error... How do I make this work?