How do I call super or object.opAssign for classes?

[Andrej Mitrovic]

class Foo
{
    void opAssign(int bar)
    {
    }
}

void main()
{
    auto foo = new Foo;
    foo = null;
}

test.d(17): Error: function test.Foo.opAssign (int bar) is not
callable using argument types (void*)
test.d(17): Error: cannot implicitly convert expression (null) of type
void* to int

I just wanted to implement one opAssign method for assigning a
specific type, but now I've ran into the issue that assigning class
objects to null doesn't work anymore.. :/