readAll() not working?

i'm doing my normal stuff with computercraft, just messing around, and when i try to do something like example.readAll() it comes back with
Attempt to index ? (a nil value)
if theres anything i did wrong please tell me, i have a post in ask a pro asking why this happens with the code. for DannySMc, it works but i'm pretty sure he uses a emulator

Please post your full code. If this is a question about another topic, I recommend asking it there as there will be a little more context.

Please don't make multiple posts for the same thing, follow this up in your Ask a Pro thread as this is definitely an issue with your code.