fetch_feed works on some feeds, not on others

The example code works fine on this RSS feed:

http://www.telegraph.co.uk/sport/rss

But not on this one, which works fine in iTunes and the RSS clients I’ve tried:

http://bigi.org.uk/podcast/feed.xml

The var_dump on the retrieved feed gives:

fetchedobject(WP_Error)#267 (2) { [“errors”]=> array(1) { [“simplepie-error”]=> array(1) { [0]=> string(59) “A feed could not be found at http://bigi.org.uk/podcast/feed.xml” } } [“error_data”]=> array(0) { } }

What’s preventing my feed from working? (As an aside, I think the error handling is incorrect here; clearly there *is* a feed at the URL given, but something *about* the feed is confusing SimplePie or WP… if the error gave more detail I may have been able to fix it myself).

Any suggestions gratefully received.