Ruby 1.9.2 ActiveResource failure (AWDwR3, section 26)

In running a scenario that works on Rails 2.3.4, and on Rails 3.0pre, both with Ruby 1.8.7, what I get with Rails 3.0pre and Ruby 1.9.2 is a number of errors:

get /orders/1/line_items.xml

<body><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
  <HEAD><TITLE>Internal Server Error</TITLE></HEAD>
  <BODY>
    <H1>Internal Server Error</H1>
    wrong number of arguments (2 for 0)
    <HR>
    <ADDRESS>
     WEBrick/1.3.1 (Ruby/1.9.2/2009-10-21) at
     127.0.0.1:3000
    </ADDRESS>
  </BODY>
</HTML>
</body>

Observations: somewhere there is a wrong number of arguments involved, but not enough information is placed into the response nor in the log to help debug. Additionally, embedding a HTML response in a is not appropriate for an XML response.