Using turf to create a union of a LayerGroup throws a TypeError

Question

I have a Leaflet LayerGroup that I add turf.buffers to like that:

const buffered = turf.buffer(trackAsJson, 0.5, {units: 'kilometers'});

After adding two tracks, the layer group looks like this:

{
  type: "FeatureCollection",
  features: Array(2)
}features: Array(2)0: geometry: {
  type: "Polygon",
  coordinates: Array(1)
}properties: {
  name: "Track 1",
  time: "2016-10-15T13:24:36.000Z"
}type: "Feature"__proto__: Object1: geometry: {
  type: "Polygon",
  coordinates: Array(1)
}properties: {
  name: "Track 2",
  time: "2018-02-17T09:58:32.000Z"
}type: "Feature"__proto__: Objectlength: 2__proto__: Array(0)type: "FeatureCollection"__proto__: Object

Now, I'd like to create a union of this LayerGroup:

unify(buffers.getLayers());


private unify(polyList): L.GeoJSON {
    let unionTemp: any;
    for (let i = 0; i < polyList.length; i++) {
      if (i === 0) {
        unionTemp = polyList[i].toGeoJSON();
      } else {
        unionTemp = turf.union(unionTemp, polyList[i]);
      }
    }
    return L.geoJSON(unionTemp);
  }

However, when I do that, I get the following error:

ERROR Error: Uncaught (in promise): TypeError: Cannot read property 'type' of undefined
TypeError: Cannot read property 'type' of undefined

In the line

    unionTemp = turf.union(unionTemp, polyList[i]);

I really don't understand why. Any hints would be welcome.