Marshal.PtrToStructure nested classes - constructor mess without parameters

Question

When marshaling an instance of a class type, the type needs a constructor without parameters. Otherwise Marshal.PtrToStructure MissingMethodException. When sorting an instance that contains nested classes, they don't need a constructor without parameters.

See the sample code below. Marshalling works correctly, output 1 2 3 4. But why DummyNesteddoesn't the class need a constructor without parameters?

static void Main(string[] args)
{
    var buffer = new byte[10];
    buffer[0] = 1;
    buffer[1] = 2;
    buffer[2] = 3;
    buffer[3] = 4;
    Dummy dummy;
    unsafe
    {
        fixed(byte* bufferPtr = &buffer[0])
        {
            dummy = (Dummy)Marshal.PtrToStructure(new IntPtr(bufferPtr), typeof(Dummy));
        }
    }
    Console.WriteLine("{0} {1} {2} {3}", dummy.m_nested1.m_data1, dummy.m_nested1.m_data2, dummy.m_nested2.m_data1, dummy.m_nested2.m_data2);
    Console.ReadKey();
}

[StructLayout(LayoutKind.Sequential, Pack = 1)]
private class DummyNested
{
    public byte m_data1;
    public byte m_data2;

    public DummyNested(byte data1, byte data2)
    {
        m_data1 = data1;
        m_data2 = data2;
    }
}

[StructLayout(LayoutKind.Sequential, Pack = 1)]
private class Dummy
{
    public DummyNested m_nested1;
    public DummyNested m_nested2;

    private Dummy()
    {
        // This one is needed to get Marshal.PtrToStructure working, but why not on DummyNested type?
    }

    public Dummy(DummyNested n1, DummyNested n2)
    {
        m_nested1 = n1;
        m_nested2 = n2;
    }
}