Restricting Template Formats in Rails 2.3.11

I spent a long time debugging a problem in Rails last night. If I
mistype a URL to may rails app (on purpose to test out the error
handling) and I put something like:

http://frogs/232.abcdef

instead of (the correct)

http://frog/232,abcdef

the abcdef becomes the "template format". i.e. instead of .json
or .xml, I am now making a request in the abcdef format. This causes
Rails to look for templates with the abcdef format so instead of
looking for a template of app/view/errors/frog.html.erb it is looking
for /app/view/errors/frog.abcdef.erb BUT, the error message doesn't
tell you this. Instead it just says "can't find app/view/errors/
frog.erb"

The route statement I'm using is just

map.resources :frogs

This creates routes that look like

/frogs/:id(.:format)

meaning that the .:format is optional. How do I tell Rails that I
don't want the (.:format) as part of my paths OR how do I tell Rails
that the :format string must match a particular set of strings? I
tried adding :format => 'html' to my list of options for that route
but that didn't have any effect.