Customized Simplified DocBook 1.1: DTD parser error : XMLconditional section not closed

Hi,

During a test-run today I discovered: 
/usr/share/xml/docbook/custom/simple/1.1/sdocbook-custom.dtd:452: parser
error : XML conditional section not closed

Unfortunately, line 452 is the last line. Can you help me? Where is the
issue?

Regards, Daniel