{
    "expression": ["index-of", ["get", "substr"], ["string", ["get", "str"]]],
    "inputs": [
      [{}, {"properties": {"substr": null, "str": "helloworld"}}],
      [{}, {"properties": {"substr": "foo", "str": "helloworld"}}],
      [{}, {"properties": {"substr": "low", "str": "helloworld"}}],
      [{}, {"properties": {"substr": "low", "str": null}}]
    ],
    "expected": {
      "compiled": {
        "result": "success",
        "isFeatureConstant": false,
        "isZoomConstant": true,
        "type": "number"
      },
      "serialized": ["index-of", ["get", "substr"], ["string", ["get", "str"]]],
      "outputs": [
        -1,
        -1,
        3,
        {"error": "Expected value to be of type string, but found null instead."}
      ]
    }
  }