"not a string" when attempting to use Kartograph.py

[zesterer]

I run:
kartograph cfg.json -f svg -o map.svg
Where the contents of cfg.json are:

{
	"proj" : {
		"id" : "mercator",
		"lon0" : -5
	},
	"layers": {
		"background" : {"special" : "sea"},
		"constituencies" : {
			"src" : "scotland.shp",
			"simplify" : true
		}
	}
}

The following error is produced:

  /cli.py, in render_map()
  71: K.generate(cfg, args.output, preview=args.preview, format=format, stylesheet=css)  /kartograph.py, in generate()
  46: _map = Map(opts, self.layerCache, format=format)  /map.py, in __init__()
  42: layer = MapLayer(layer_id, layer_cfg, me, layerCache)  /maplayer.py, in __init__()
  35: self.source = handle_layer_source(self.options, self.cache)  /layersource/__init__.py, in handle_layer_source()
  32: src = ShapefileLayer(src)  /layersource/shplayer.py, in __init__()
  37: if srs.ImportFromWkt(prj_text):  /usr/lib/python2.7/site-packages/osgeo/osr.py, in ImportFromWkt()
  1108: return _osr.SpatialReference_ImportFromWkt(self, *args)
not a string%