.NET remoting, custom sink provider.

Question

When implementing IClientChannelSinkProvider's CreateSink method:

public IClientChannelSink CreateSink(IChannelSender channel, String url, Object remoteChannelData)
{

        IClientChannelSink nextSink = null;
        if (_next != null)
        {
            nextSink = _next.CreateSink(channel, url, remoteChannelData);
            if (nextSink == null)
                return null;
        }

        return new MyClientChannelSink(nextSink);
}

In what scenario _next.CreateSink returns null?