Error when using openmap() with specific tile provider URLs

[philament]

Hi, I think there is an issue in the openmap() function when type specifies a tile URL that contains transparencies.

This works fine:
toner = OpenStreetMap::openmap(c(48,11), c(47,10), zoom=8, type = http://tile.stamen.com/toner/{z}/{x}/{y}.png')

However, the following, which uses only a slight different style of the same map from the same provider (toner-lines instead of toner) fails:

toner = OpenStreetMap::openmap(c(48,11), c(47,10), zoom=8, type = http://tile.stamen.com/toner-lines/{z}/{x}/{y}.png')

Error in col2rgb(x$colorData) : invalid RGB specification

I think it might be related to transparent areas in the returned tiles? Could it be as simple as adding alpha = TRUE to the call to col2rgb() in openmap()?