DMD different compiler behaviour on Linux and Windows

[Zans via Digitalmars-d-learn]

import std.stdio;

void main()
{
    char[] mychars;
    mychars ~= 'a';
    long index = 0L;
    writeln(mychars[index]);
}
Why would the code above compile perfectly on Linux (Ubuntu 
16.04), however it would produce the following error on Windows 
10:

source\app.d(8,21): Error: cannot implicitly convert expression 
index of type long to uint

On both operating systems DMD version is 2.085.0.