convert string to ubyte[]

[aki via Digitalmars-d-learn]

Hello,

This will be trivial question but I cannot figure out
what's wrong. I want to convert string to an array of ubyte.

import std.conv;
void main() {
        auto s = "hello";
        ubyte[] b = to!(ubyte[])(s);
}

It compiles but cause run time error:

std.conv.ConvException@C:\APP\D\dmd2\windows\bin\..\..\src\phobos\std\conv.d(3530): Can't 
parse string: "[" is missing

I cannot understand the meaning of this message.
Replacing s with s.dup to remove immutable doesn't help.
Do I need to use cast?

Regards,
Aki