switch statement with variable branches

[Yuxuan Shui via Digitalmars-d-learn]

Somehow I can't use ubyte variables behind 'case', but ulong 
works fine. Why is that?

void main() {
        alias TestType = ulong; // won't compile if = ubyte
        import std.stdio;
        TestType a,b,c;
        readf("%s %s %s ", &a, &b, &c);
        switch(c){
                case a: writeln("a");break;
                case b: writeln("b");break;
                default: assert(false);
        }
}