'Use Map Skew' Doesn't Work on Server

It looks like when the app is hosted on the server, if the user checks the 'Use Map Skew' option for any sites on the first tab it will give an error:

I'm guessing this is related to something with file paths accessing the resource files supplied with the package. The relevant files (one shapefile and two geotiffs) are in the inst/extdata folder of the package.

The 'Use Map Skew' option should work for most of the US that isn't California or northern Alaska. Site 12340000 is one where selecting 'Use Map Skew' should successfully return a value.