Nil

Nil is a special value denoting the absence of a value, and has the type nil. It is often returned by functions to indicate an error condition, usually along with an error message as the second return value (e.g. loadfile, loadstring). It is good practice to check if a variable is nil before using it, to prevent errors such as "attempt to call nil" or "attempt to index ? (a nil value)", which will happen when a nil value is used regardless.