Redirect loop / too many redirects

[TinyApps.Org]

I am trying to redirect all requests for

/weblog/index.rss

to

/weblog/index.rss20

with this rule:

location /weblog/index.rss { rewrite /weblog/index.rss /weblog/ 
index.rss20 permanent; }

but I end up with an error message in the browser like:

Safari: Too many redirects occurred trying to open “feed://example.org/weblog/index.rss 
”. This might occur if you open a page that is redirected to open  
another page which then is redirected to open the original page.

Firefox: Redirect Loop - Firefox has detected that the server is  
redirecting the request for this address in a way that will never  
complete.

All other redirect rules work - any idea why this one would fail?