opCmp with structs

[Alex via Digitalmars-d-learn]

I'm sure I'm doing a silly mistake somewhere, but why this 
doesn't work?
import std.stdio;
import std.algorithm;

struct ku
{
        immutable int id;
        alias id this;
        
        this(int i)
        {
                id = i;
        }
        
        int opCmp(ref const ku rhs) const {return id - rhs.id; }
}
void main()
{
        ku[] tt = [ku(2), ku(1)];
        sort(tt);
}