following-sibling usage

Hello there,

I have a problem while using the following-sibling axis. Here's an 
simplified example of my XML data :

  <FICHE cdId="663" catanetId="13343" type="P" lang="FRA">
    <TITRE>140 000 CHINOIS POUR LA GRANDE GUERRE</TITRE>
    <THEME>Histoire</THEME>
    <SOUSTHEME>Histoire Contemporaine</SOUSTHEME>
    <GENERIQUE>
      <GENERAL>
        <ENTREE lib="Durée">
          <VALEUR>52 min</VALEUR>
        </ENTREE>
<!-- MORE SUBTAGS HERE -->
      </GENERAL>
<!-- MORE SUBTAGS HERE -->
  </FICHE>

I want to make a test on the following sibling SOUSTHEME (subtag of 
FICHE), as my test always returned FALSE, I decided to write the data.
Here's what I got in my xsl:for-each select="FICHE" :

current : <xsl:value-of select="SOUSTHEME" />
following : <xsl:value-of select="following-sibling::SOUSTHEME[1]" />

And here's the output :

current : Biographie 
following :

As you see, this expression failed : 
select="following-sibling::SOUSTHEME[1]". How should I use the following 
sibling axis... I can't find my error ! :/

Any help would be appreciated. Thanks !

Simon

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