RedBlackTree thin wrapper initialization

[BlackEdder via Digitalmars-d]

I'm trying to write a thin wrapper around redblacktree, but it 
seems every object of the class shares the same copy of 
redblacktree. Am I doing something wrong or is this a bug.

Minimal code example:
import std.array;
import std.container;
import std.stdio;

class A {
    auto tree = new RedBlackTree!string();
}

unittest {
    auto a = new A();
    a.tree.insert( "a" );
    auto b = new A();
    writeln( "Should be empty, but is: ", b.tree.array );
    writeln( "Should be empty, but has length: ", b.tree.length );
}

Which results in the following output:
$ rdmd -unittest rbt.d
Should be empty, but is: ["a"]
Should be empty, but has length: 1


I'm using dmd 2.0.65.