Why can't I subscribtion a xml that project original create?

http://kaiping.in/forums/2/topics.xml

I can read this in firefox 3.50 but google reader can’t.
it throw me a error message:

Your search did not match any feeds.

Suggestions

* Make sure all words are spelled correctly.
* Try different keywords.
* Try more general keywords.

I know it doesn’t related to rails. but I don’t know where to go.

Zhenning G. wrote:

http://kaiping.in/forums/2/topics.xml

I can read this in firefox 3.50 but google reader can’t.
it throw me a error message:

Your search did not match any feeds.

Well, I’d say because it’s not a properly formatted RSS or ATOM feed. I
just clicked your link in Safari (built-in RSS/ATOM reader) and it won’t
work there either. I think you need to compare what you’re generating
(including all necessary HTTP headers) with the RSS/ATOM specs.