XSLT error: The expression does not evaluate to a node

Hi all, 

I have the following code snippet: 


code:--------------------------------------------------------------------------------
  <xsl:template name="comparePanels">
    <!-- compare two panels of a form -->
    <xsl:param name="panels_a" />
    <xsl:param name="panels_b" />

    <xsl:if test="$panels_a | $panels_b">
      <xsl:variable name="compar">
        <xsl:call-template name="comparePanelTitle">
          <xsl:with-param name="panel_a" select="$panels_a[1]"/>
          <xsl:with-param name="panel_b" select="$panels_b[1]"/>
        </xsl:call-template>
      </xsl:variable>

      <xsl:value-of select="$compar"/>

      <xsl:choose>
        <xsl:when test="$compar='equal'">
          <xsl:element name="{name($panels_a[1])}">
            <xsl:attribute name="Title1">
              <xsl:value-of select="$panels_a[1]/@Title"/>
            </xsl:attribute>
            <xsl:attribute name="Title2">
              <xsl:value-of select="$panels_b[1]/@Title"/>
            </xsl:attribute>
            <xsl:attribute name="Result">
              <xsl:text>Labels are equal</xsl:text>
            </xsl:attribute>
          </xsl:element>
<!--
          <xsl:call-template name="comparePanels">
            <xsl:with-param name="panel_a" select="$panels_a[following-sibling::*]"/>
            <xsl:with-param name="panel_b" select="$panels_b[following-sibling::*]"/>
          </xsl:call-template>
-->
        </xsl:when>
--------------------------------------------------------------------------------

if I delete "<!--" and "-->" I get an error message (with xalan): 

code:--------------------------------------------------------------------------------
XSLT error: The expression does not evaluate to a node-set
--------------------------------------------------------------------------------

I don't understand why because the expresion is a node-set but without the first node or am I wrong?
What can I do that the program works?

Thanks in advance,

Oliver
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