Attempt to concatenate string and nil error

ok, so Im setting up a security interface for a dungeon on a server, and its suppose to either read a keycard (floppy disc) or input from the user.

Code giving the error:

clearance = "/"..disk.getMountPath("bottom").."/"..keycode
fs.open(clearance,"r")

And is that all the code, or…?

disk.getMountPath will return nil if there is not a floppy disk in the disk drive. You should first use disk.hasData to determine if there is a floppy disk in the indicated location.