Added nbs extrap.box values and changed function default value of extrap.box to NULL, which can be a little easier to handle than is.na(extrap.box)[1] because is.null(extrap.box) will simply be FALSE regardless of what extrap.box is as long as it is not actually NULL. Also, I wonder if this exact script was not pushed out of the dev branch (assuming you have one?) because when I explored the function in R after a fresh download of the package, it read "if (is.na(extrap.box)) {" without the "[1]", which was present here.
Added nbs extrap.box values and changed function default value of extrap.box to NULL, which can be a little easier to handle than is.na(extrap.box)[1] because is.null(extrap.box) will simply be FALSE regardless of what extrap.box is as long as it is not actually NULL. Also, I wonder if this exact script was not pushed out of the dev branch (assuming you have one?) because when I explored the function in R after a fresh download of the package, it read "if (is.na(extrap.box)) {" without the "[1]", which was present here.