class opBinary overloading. value + null and null + value

[ref2401]

public class MyClass
{
	private int value;

	this(int val)
	{
		value = val;
	}

	MyClass opBinary(string op: "+")(MyClass rhs)
	{
		if (rhs is null)
		{
			return null;
		}

		MyClass result = new MyClass(value + rhs.value);
		return result;
	}
}

void main()
{
	MyClass v1 = new MyClass(4);
	MyClass v2 = new MyClass(5);

	auto v3 = v1 + null; // v3 is null
	auto v4 = null + v2; // v4 equals v2
}

in this example v3 is null. i thought v4 would be null too, but i 
was wrong.
what should i do to make my binary operation commutative?