C# script in XSLT: "msxsl:script cannot be empty" error

I have an XSLT with a C# script in it:

<msxsl:script language="C#" implements-prefix="user">
  <msxsl:using namespace="System.Xml"/>
  <msxsl:using namespace="System.Xml.XPath"/>
  <![CDATA[
    public string GetMenuPath(XPathNodeIterator node)
    {       
      XPathNavigator dataSourceNode = node.Current;
      return dataSourceNode.Name;
    }
  ]]>
</msxsl:script>

And somewhere in the XSLT this function is called:

<xsl:value-of select="user:GetMenuPath(current())"/>

But as I try to run the XSLT transformation on the XML input (the input does not matter), it stop immediately with the error "msxsl:script cannot be empty".

I have absolutely no idea why... I use nxslt with .NET Framework.

Thank you for help !