DOCBOOK-APPS: Error: Output character not available

I've got Saxon 6.4.3 working with the XSL DocBook stylesheets, and on some
XML source files produce nice HTML Help. However, on a large docset I
get the following error:

Error on line 117 of
file:/C:/tools/docbook/docbook-xsl-1.45/html/chunker.xsl:
  Output character not available in this encoding (decimal 8217)
Transformation failed: run-time errors were reported

My XML files are encoded in UTF-8, so I don't understand what's going on
here. Any ideas?

TIA

Gershon