Caesar Cipher

[Mario via Digitalmars-d-learn]

Hello there! I know deep Java, JavaScript, PHP, etc. but as you 
all probably know, that's high-level and most of them only use 
the heap memory.

So I'm new to the wonderful world of low-level and the 
stack-heap. I started a week ago learning D (which by the moment 
is being easy for me) but I'm facing a big problem: I don't know 
how to do the exercise of 
https://tour.dlang.org/tour/en/basics/arrays . I really have been 
looking on forums and Google but I found this in Java 
https://stackoverflow.com/questions/10023818/shift-character-in-alphabet which is actually not the best due to Java uses other ways.

My code is something like this:

char[] encrypt(char[] input, char shift)
{
    auto result = input.dup;
    result[] += shift;
    return result;
}


What's wrong? I mean, I know that z is being converted into a 
symbol, but how should I fix this?
Thanks on forward.