[xsl] Unwanted namespace after using COPY-OF

[gerritjan <gerritjankoekkoek@xxxxxxx>]

Newbe question?
This is my XSLT:

<?xml version="1.0"?>
<xsl:stylesheet version="1.0"
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform";
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance";
       exclude-result-prefixes="xsi"
    >
  <xsl:output method = "xml" indent = "yes" />

  <xsl:template match="/" >
    <onderwerpen>
       <xsl:apply-templates select="onderwerp">
    </onderwerpen>
  </xsl:template>

  <xsl:template match="onderwerp" >
    <xsl:variable name="v_titel"><xsl:value-of 
select="@titel"/></xsl:variable>
    <xsl:variable name="v_datum"><xsl:value-of 
select="@datum"/></xsl:variable>
    <xsl:variable name="v_datum_einde"><xsl:value-of 
select="@datum_einde"/></xsl:variable>
    <xsl:variable name="v_menu"><xsl:value-of 
select="@menu"/></xsl:variable>
    <xsl:variable name="v_submenu"><xsl:value-of 
select="@sub_menu"/></xsl:variable>
    <xsl:variable name="v_beveiliging"><xsl:value-of 
select="@beveiliging"/></xsl:variable>
	<onderwerp titel="{$v_titel}" datum="{$v_datum}" 
datum_einde="{$v_datum_einde}" menu="{$v_menu}" submenu="{$v_submenu}" 
beveiliging="{$v_beveiliging}" >
      			<xsl:copy-of select="subtitel"/>
      			<xsl:copy-of select="kenwoorden"/>
      			<xsl:copy-of select="over_cdls_persoon"/>
      			<xsl:copy-of select="inleidingtekst"/>
      			<xsl:copy-of select="onderwerptekst"/>
      			<xsl:copy-of select="auteur"/>
      			<xsl:copy-of select="bron"/>
	</onderwerp>
  </xsl:template>

</xsl:stylesheet>

The result contains:
<kenwoorden xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance";>
<inleidingtekst xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance";>

It looks like using <XSL:COPY-OF...> is adding this namespacestatement, 
I did not intend this "xmlns:xsi..." to be added!
I try to suppress it by the 2 lines added to the:
<xsl:stylesheet version="1.0"
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform";
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance";
       exclude-result-prefixes="xsi">

This does not help?
Any suggestions?

Gerritjan Koekkoek