clear() causes crash?

[Luís.Marques]

This crashes in the last line of main:

    class A
    {
        void foo() {}
    }
    void main()
    {
        A a = new A();
        a.foo();
        clear(a);
        assert(a !is null);
        a.foo();  // crashes
    }

As far as I understand from TDPL book, this should not crash, but 
it does (DMD64 v2.062, OS X). Am I misunderstanding clear()?

BTW, why not make clear also change 'a' to null?