Charset error

[har777]

I keep getting this error :

charset=layer.options['charset']
KeyError: 'charset

My config.json:

{
    "proj": {
        "id": "mercator",
        "lon0": 20
    },
    "layers": [
        {
            "special": "sea",
            "styles": {
                "fill": "lightblue"
            }
        },
        {
            "special": "graticule",
            "latitudes": 10,
            "longitudes": 10,
            "styles": {
                "stroke-width": "0.4px"
            }
        },
        {
            "id": "countries",
            "src": "country_boundaries/ne_50m_admin_0_countries.shp",
            "attributes": {
                "ISO_A3": "iso",
                "NAME": "name",
                "POP_EST": "population",
                "GDP_MD_EST": "gdp"
            }
        }
    ],
    "export": {
        "width": 800,
        "height": "auto"
    },
    "bounds": {
        "padding": 0.02
    }
}