Relax-NG -> DTD with Trang?

I use trang to produce DTD from Relax-NG schemas but it fails when the
Relax-NG schema is a Docbook extension of the Docbook schema provided
with nxml:

% trang -I rnc -O dtd codevnic-docbook.rnc codevnic-docbook.dtd
/usr/share/emacs/site-lisp/nxml-mode/schema/calstbl.rnc:87:1: error: multiple definitions of "#start" without a "combine" attribute
/usr/share/emacs/site-lisp/nxml-mode/schema/calstbl.rnc:174:1: error: sorry, combining definitions with combine="choice" is not supported
...