The error I get when I run your code is:

System.ArgumentException: 'ISO-8559-1' is not a supported encoding name.

It's thrown by the standard .NET Framework encoding classes. It means the page declares an encoding not supported by .NET. I fixed it like this:

var page = new HtmlDocument();
page.OptionReadEncoding = false;

PS: I'm using the Html Agility Pack version 1.3