For some obscure reason, a variable pkgPath is returned as character(0) in my unit tests run on R-devel (win10) under CMD check (in interactive session they run OK, as well as on R-3.6.1). Anyway, if this line:

returns character(0) the .linkingToIncludes() ends up with an error "argument is of length zero" in the lineif (file.exists(pkgHeaderPath)) {. It would be great to add a test forlength(pkgPath) and if it is zero, to stop with a meaningfull error message "package 'pkg' is not found" or alike.