Vega: Projection property fit "Unsupported parameter object" error

gistfile1.txt

{
  "$schema": "https://vega.github.io/schema/vega/v3.json",
  "width": 960,
  "height": 500,
  "autosize": "none",

  "data": [
    {
      "name": "unemp",
      "url": "data/unemployment.tsv",
      "format": {"type": "tsv", "parse": "auto"}
    },
    {
      "name": "counties",
      "url": "data/us-10m.json",
      "format": {"type": "topojson", "feature": "counties"},
      "transform": [
        { "type": "lookup", "from": "unemp", "key": "id", "fields": ["id"], "values": ["rate"] },
        { "type": "filter", "expr": "datum.rate != null" }
      ]
    }
  ],

  "projections": [
    {
      "name": "projection",
      "type": "albersUsa",
      "fit": {}
    }
  ],

  "scales": [
    {
      "name": "color",
      "type": "quantize",
      "domain": [0, 0.15],
      "range": {"scheme": "blues-9"}
    }
  ],

  "legends": [
    {
      "fill": "color",
      "orient": "bottom-right",
      "title": "Unemployment",
      "format": "0.1%",
      "encode": {
        "symbols": {
          "update": {
            "shape": {"value": "square"},
            "stroke": {"value": "#ccc"},
            "strokeWidth": {"value": 0.2}
          }
        }
      }
    }
  ],

  "marks": [
    {
      "type": "shape",
      "from": {"data": "counties"},
      "encode": {
        "enter": { "tooltip": {"signal": "format(datum.rate, '0.1%')"}},
        "update": { "fill": {"scale": "color", "field": "rate"} },
        "hover": { "fill": {"value": "red"} }
      },
      "transform": [
        { "type": "geoshape", "projection": "projection" }
      ]
    }
  ]
}