Aloha all,

Answering myself.  This bug is known and was discussed several years 
ago:

 From: Myles English
Subject: Re: [O] babel R: should/does order of parameters matter?
Date: Tue, 29 Mar 2011 16:57:37 +0100

Apparently, ob-R wants tables to be passed to parameters that are early 
in the parameter list.

Also, ob-R expects all the cells in a table to be non-empty.  An empty 
table cell raises Cain.

Argh.

All the best,
Tom