Source url could be undefined -- check before calling .split or performing other operations on it

[jseppi]

On this line in _getAttribution:

mapbox.js/src/style_layer.js

Line 33 in 1db88e7

var source = style.sources[id].url.split('mapbox://')[1];

Each source url has .split() called on it. If a source without a url property (such as a geojson source) is present in the style, this line will cause an uncaught error.

[jseppi]

Potentially the same issue as #1295