Cannot use result tree fragment

[abbouh <ab...@ra...>]

i want to get all "ht"  nodes from different  external documents,so i
use in my xsl file:

<xsl:for-each select="//doccontent/chapmod">
<xsl:variable name="vchapmod" select="current()"/>
<xsl:variable name="vchapmod1">
<xsl:value-of
select="document(concat(string($vchapmod/attribute::docref),'.xml'))"/>
</xsl:variable>
<xsl:for-each select="$vchapmod1//ht">

but i get this error :
Error in Xpath expression,Cannot use result tree fragment
the error is for <xsl:for-each select="$vchapmod1//ht">

why , and any other suggestion
Thanks