Missing tiles in French Alps

[Wivik]

Hello,

I'm integrating the toner-background map to use actual maps for a game, but the tiles are missing while zooming.

Here's the used location : 45.98479/6.93342

http://maps.stamen.com/toner-background/#15/45.98479/6.93342

Also here : 46.00188/6.94634

http://maps.stamen.com/toner-background/#15/46.0019/6.9463

Aside this issue, the rendering is just perfect for my needs, thanks for your work 👍