string.sub question

So I have a program that needs to take a string received through rednet, and determine what the last few letters are.
I have set up a string.sub to handle this that looks like this:

id, message = rednet.receive()
messagesub = string.sub(message, pos1, pos2)

I've tried:

local messagelength = #message - 4 –subtract for from the total length
message = string.sub(message, messagelength)

However, its keeps giving me an error, "attempt to get length of nil"

what I was wondering is if there is a way to get the last 4 letters of the string. Any help is appreciated!

Thanks for the reply! trying it now

Why thank you good sir. It works!

Or even…

message:sub( -4 )

Your "attempt to get length of nil" error would suggest that 'message' was nil, however, so when you tried "#message", it errored.