respond_with fails silently on missing mimetype

Happens on a 'PUT /things/1' request with the below code; haven't tried with POST or any other verbs at the moment.

class ThingsController < ApplicationController

respond_to :html, :xml, :json

def update
    @thing = Thing.find(params[:id])
    @thing.update_attributes(params)
    @thing.save!
    respond_with(@thing)
end

end

If I execute a PUT request with anything but the default format, the above code fails silently; nothing in the logs, nothing fired back to the user. If I replace the 'respond_with' block with an older 'respond_to do |format|' style block, or manually add the mimetypes in via a block passed to respond_with, then things work again.

By 'anything but the default format', I mean that if I append a format to the URL, things fail silently. This happens with '.xml', '.json', '.totallyinvalidformat', etc.