xml document as parameter and attribute expression

Hi,

In my XSLT stylesheet, I have a global parameter containing some XML
content like this :

<div id="center" title="myTitle">
...
</div>

This parameter is called 'center' and I would like to retrieve the
value of title with some XPath expression. I tried the following :

<xsl:value-of select="$center/div[@id='center']/@title"/>

But my libxslt/php5 XSLT processor gives me an XPath internal error
(document without root) and I don't understand what's the problem.
BTW I have another stylesheet that does nearly the same  except it
receives the uri of a document as a parameter and uses
document($center) instead of center.

I guess it's a problem of tree parsing but how can I solve that,
knowing that I can't change the fact that I receive XML string
directly as a parameter (XSLT transform streaming) ?

Thx in advance.

--
Sebastien ARBOGAST