Cropped layer in Kartograph.py for SVG maps

Question

I'm trying to draw a SVG map with two layers using Kartograph.py. The end result should be a single country on the first layer (say France) with the main rivers on a second layer.

The first layer with France borders works perfectly, but then I would like to add additional topological information on top of the map (say rivers). I have found the shapefile describing the rivers. However, when I add it in the JSON configuration file as shown below, the rivers are out of the country (but inside the square defined by the country, see image).

The documention of Kartograph mentions crop-to, but I cannot get it to work. When I add this option to crop the second layer to the first one, all the river data does not appear.

{
    "proj": { 
        "id": "laea"
    },
    "layers": [
        {
            "id": "layer0",
            "src": "FRA_adm2.shp",
            "simplify": 2
        },
        {
            "id": "rivers",
            "src": "rivers.shp",
            "crop-to": "layer0", /* This is not working */
            "simplify": 0
        }
    ]
}