How to test if the variable contains nodeset or not

Hello,
I am using Saxon and stylesheet version 1.1 and I regularly pass node-sets
as parameters to xsl:call-template.

Now I need to test, if a parameter within called template contains nodeset
or it contains string, because when it is not handled I get exception > The
value is not a node-set

  <xsl:template name="normalize-messages">
    <xsl:param name="messages"/>

    <xsl:choose>
      <xsl:when test="$messages/tr/td">
        ...
      </xsl:when>
    </xsl:choose>
  </xsl:template>
...
   <xsl:call-template name="normalize-messages">
      <xsl:with-param name="messages">
        <tr>
           <td>Message type 1</td>
        </tr>
      </xsl:with-param>
       or 
      <xsl:with-param name="messages" select="'Messsage type 2'"/>
   </xsl:call-template>

I found similar problem on XSLT FAQ, but it does not provide solution.
http://www.dpawson.co.uk/xsl/sect2/N8090.html#d8048e946 

Thank you,
Michal

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