I need to navigate through this xml tree.

<publication>
    <corporate>
        <contentItem>
            <metadata>meta</metadata>
            <content>html</content>
        </contentItem>
        <contentItem >
            <metadata>meta1</metadata>
            <content>html1</content>
        </contentItem>
    </corporate>
    <eurasia-and-africa>
       ... 
    </eurasia-and-africa>
    <europe>
       ...
    </europe>
</publication>

and convert it to html with this stylesheet

 <ul>
     <xsl:variable name="itemsNode" select="concat('publicationManifest/',$group,'/contentItem')"></xsl:variable>
     <xsl:for-each select="$itemsNode">
         <li>
        <xsl:value-of select="content"/>
         </li>
    </xsl:for-each>
</ul>

$group is a parameter with name of group for example "corporate". I have an error with compilation of this stylsheet. SystemID: D:\1\contentsTransform.xslt

Engine name: Saxon6.5.5
Severity: error
Description: The value is not a node-set
Start location: 18:0

What the matter?