Emacs, nsgml and DocBook

Hi every one,

I'm a neewbie with XML and DocBook. I'm using Emacs with the psgml-mode. 
  When I want to validate a simple file, I have errors like this :
nsgmls -s  truc.xml
nsgmls:<URL>http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd:74:17:E: 
"X20AC" is not a function name
nsgmls:<URL>http://www.oasis-open.org/docbook/xml/4.1.2/ent/iso-amsa.ent:8:19:E: 
"X21B6" is not a function name
nsgmls:<URL>http://www.oasis-open.org/docbook/xml/4.1.2/ent/iso-amsa.ent:9:19:E: 
"X21B7" is not a function name
nsgmls:<URL>http://www.oasis-open.org/docbook/xml/4.1.2/ent/iso-amsa.ent:10:17:E: 
"X21D3" is not a function name
nsgmls:<URL>http://www.oasis-open.org/docbook/xml/4.1.2/ent/iso-amsa.ent:11:18:E: 
"X21CA" is not a function name
	(...)

Here is the file I try to validate :
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN" 
       "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd">
<article>
   <section>
     <title>hbjhhj</title>
     <para>hjjh</para>
   </section>
</article>

What's wrong ?

Thanks (for your help and for enduring my Frenchman English ;-)
Christophe