How can I conditionaly assign a node-set to my variabl

How can I conditionaly assign a node-set to my variable? 
Variable needs to have local scope. 
Obviously I can't use <xsl:value-of.../>. 
I get the error message: 
"Reference to variable or parameter 'firstTableRows' must 
evaluate to a node list." 

Code: 

<xsl:variable name="firstTableRows"> 
 <xsl:choose> 
   <xsl:when test="6&gt;$n and $hasP"> 
     <xsl:value-of select="channel[6&gt;id or id='P']"/> 
   </xsl:when> 
   <xsl:when test="6&gt;$n"> 
     <xsl:value-of select="channel[6&gt;id]"/> 
   </xsl:when> 
   <xsl:otherwise> 
     <xsl:value-of select="channel[6&gt;id]"/> 
   </xsl:otherwise> 
 </xsl:choose> 
</xsl:variable> 

Thanks, 
Roy