HTML Agility Pack - 只能从文件系统加载xml文档，而不能从Web加载

I've used HAP successfully before, downloading xhtml pages from web. However, now I'm trying to load and parse xml documents. HAP will only load xml documents that are located on my file system, "C:\xml\MyXml.xml" for instance. It will not load it from web (https://www.web.com/doc.xml). Using Fiddler, I can see that HAP is actually requesting the xml documents over the web, and the server also responds with the xml document. However, it stops there, nothing get parsed. The HtmlDocument is empty, no ChildNodes or anything. When loading from file system, it get parsed successfully to a HtmlDocument.I've used HAP successfully before, downloading