DOCBOOK-APPS: expected markup declaration?!

I tried to add a customization layer to the DocBook XML DTD, which seemed to
have worked reasonably well until I tried to run an authored document
through Saxon to produce HTML output using Norm's stylesheets. Then I got an
error: "expected markup declaration". Any idea what might be wrong here? The
line indicated is #2 (with the DOCTYPE declaration & DTD reference), but the
column indicated is nonsense. The problem obviously lies with the
customization driver file, though as far as I can tell, I adhered to the
guidelines described in the documentation, and validation seems to work
fine.