xsl for two type of document

i have two type of  input document(foprdoc and genericdoc)
and i want to use one xsl file for them.
and  i don't want to test each time if the input file is a  foprdoc or
genericdoc
(because i must use foprdoc or genericdoc in my xpath expressions)
so i use a variable like:
<xsl:variable name="racine">
  <xsl:choose>
  <xsl:when test="/genericdoc">
  <xsl:value-of select="/genericdoc"/>
  </xsl:when>
  <xsl:when test="/foprdoc">
  <xsl:value-of select="/foprdoc"/>
  </xsl:when>
  </xsl:choose>
</xsl:variable>

and i call
<xsl:apply-templates select="$racine/doccontent/docbody"/>

but i receive this error:
XSLT Error (javax.xml.transform.TransformerException):
java.lang.ClassCastException: org.apache.xpath.objects.XRTreeFrag



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