string.format a nil value

["jason zhang" <jzhang@...>]

if I pass a nil value to string.format function, it will raise a error and the interpreter quit.

For example,

 

string.format("%d",novalue), where novalue is a nil variant.

 

Why not outputing a "NIL" like string? It think it's not very friendly to print a error information and

exit.

 

FYI

Thank you.